[transport] usb: Fix htool crash on USB disconnect

[wak-google]

Previously, libhoth_usb_fifo_run_transfers() could return while one or more libusb transfers were still pending. This happened if the second transfer submission failed, or if the event handling loop was interrupted by a signal (e.g. SIGINT).

When htool subsequently attempted to close and reopen the transport, it would call libusb_free_transfer() on these pending transfers, triggering an assertion failure in libusb:
'usbi_mutex_lock: Assertion pthread_mutex_lock(mutex) == 0 failed'

This change ensures that:

1. All submitted transfers are completed (success, error, or cancel) before the function returns.
2. If a transfer submission fails, any other successfully submitted transfer is cancelled and waited for.
3. Signal interruptions do not cause an early return while transfers are pending.
4. Completion flags are correctly initialized on open.

Verified on yutulis-ru4-bmc-01 with 1000 iterations of target reset spam without a crash.