tune TinyUSB fail wait delay to 15ms

Previously, I used a 50ms delay after hitting a TinyUSB tuh_* API
call failure result to have a big safety margin. But, that seems
long enough that it might mess up animation loops. So, I tried
many builds with delays between 0ms and 20ms, testing unplug
detection with my big pile of USB devices.

A 15ms delay worked fine with everything I tried. My Full Speed
devices were marginal down to 13ms, but they stopped working at
12ms. Some of my Low Speed devices were okay down to 3ms, but the
Adafruit generic SNES gamepad was marginal below 12ms.

Author: samblenny

shared-module/usb/core/Device.c

@@ -69,10 +69,17 @@ void common_hal_usb_core_device_deinit(usb_core_device_obj_t *self) {
 }
 
 static void _wait_then_raise_USBError(int errno) {
-    // 1. Spin for 50 ms in a background task loop because some USB glitches
-    //    will magically clear up if you just wait for a bit
-    // 2. Raise the exception so the calling code knows it needs to try again
-    const uint32_t end = supervisor_ticks_ms32() + 50;
+    // 1. Spin briefly in background task loop because this seems to fix the
+    //    enumeration or addressing glitch that can cause tuh_* calls to fail after
+    //    a device is unplugged. Don't know why this works, but things magically
+    //    self correct if you just wait for a bit before attempting another control
+    //    transfer.
+    // 2. Raise the exception so the calling code knows it needs to try again.
+    //
+    // TODO: Consider removing this mechanism if somebody finds and fixes whatever is
+    // causing TinyUSB to get confused about unplugged devices.
+    //
+    const uint32_t end = supervisor_ticks_ms32() + USB_CORE_TUH_FAIL_WAIT_MS;
     while (supervisor_ticks_ms32() < end && !mp_hal_is_interrupted()) {
         RUN_BACKGROUND_TASKS;
     }

shared-module/usb/core/Device.h

@@ -19,6 +19,13 @@ typedef struct {
 } usb_core_device_obj_t;
 
 
+// Number of ms to wait in a background task loop after a TinyUSB tuh_* api call
+// returns a failure result. This delay is part of a kludge to help TinyUSB
+// recognize when a device has been unplugged. Without the delay, usb.core.find()
+// can get super broken. This mechanism can be removed once somebody finds and
+// fixes whatever is causing TinyUSB to get confused about unplugged devices.
+#define USB_CORE_TUH_FAIL_WAIT_MS (15)
+
 // These values get used to set USBError.errno and USBTimeoutError.errno.
 // It would be possible to define these to more closely mimic desktop PyUSB on
 // a given OS (maybe Debian?). But, for USB error handling in CircuitPython,