Handling the USB message queue is done in an ISR and take quite some time.
This can cause a USART/SIM sniffing buffer overrun, resulting in data loss.
By setting the USB IRQ lower than the USART IRQ, the USB ISR can be
interrupted (for short) and no data gets lost.
Change-Id: I870a0aa8e251bbb53249c54bfcaa45de5b5a9486