usb: fix DUSB Request-to-Send var type attribute header byte. Fix #509

The OS does not want some newer vartypes (like exact-math ones)
to have an "older" vartype attr "header" byte (like 0x07), making
the transfer fail. We use 0x0F as this makes all CE vars transferable.
Verified with various files to work as expected.

See also debrouxl/tilibs#90 for more details
and a less CE-specific fix.

Author: adriweb

core/usb/dusb.c

@@ -1505,7 +1505,7 @@ int usb_dusb_device(usb_event_t *event) {
                     *buffer++ = DUSB_ATTR_VAR_TYPE_SIZE >> 8 & 0xFF;
                     *buffer++ = DUSB_ATTR_VAR_TYPE_SIZE >> 0 & 0xFF;
                     *buffer++ = 0xF0;
-                    *buffer++ = command->vartype == CALC_VAR_TYPE_FLASH_APP ? 0x0F : 0x07;
+                    *buffer++ = 0x0F; // this technically depends on the OwnerPID, but we only care about the CE here...
                     *buffer++ = 0;
                     *buffer++ = command->vartype;