Check usb_busy up front in usb background function.

Waiting to do so risks accidentally queueing another response.

Hopefully fixes #655 but we'll let @jerryneedell confirm.

Author: tannewt

ports/atmel-samd/usb_mass_storage.c

@@ -280,15 +280,22 @@ int32_t usb_msc_xfer_done(uint8_t lun) {
 // drive into our cache and trigger the USB DMA to output the
 // sector. Once the sector is transmitted, xfer_done will be called.
 void usb_msc_background(void) {
-    if (active_read && !usb_busy) {
+    // Check USB busy first because we never want to queue another transfer if it is. Checking
+    // active_read or active_write first leaves the possibility that they are true, an xfer done
+    // interrupt occurs (setting them false), turning off usb_busy and causing us to queue a
+    // spurious transfer.
+    if (usb_busy) {
+        return;
+    }
+    if (active_read) {
         fs_user_mount_t * vfs = get_vfs(active_lun);
         disk_read(vfs, sector_buffer, active_addr, 1);
         CRITICAL_SECTION_ENTER();
         int32_t result = mscdf_xfer_blocks(true, sector_buffer, 1);
         usb_busy = result == ERR_NONE;
         CRITICAL_SECTION_LEAVE();
     }
-    if (active_write && !usb_busy) {
+    if (active_write) {
         if (sector_loaded) {
             fs_user_mount_t * vfs = get_vfs(active_lun);
             disk_write(vfs, sector_buffer, active_addr, 1);