firmware: protect uart_tx_queue against ISR
cardem:
- dispatch_usb_command_cardem() appends to uart_tx_queue from the main loop
- tx_byte_tpdu() dequeues from the USART IRQ handler @ NVIC prio 0
card_handle_reset() has the same issue, drains queue and
frees uart_tx_msg from main loop while the ISR may own them.
All of this needs protection against the irq.
Needs a fixed llist_add_tail_irqsafe(), which called __enable_irq() instead of
restoring the saved PRIMASK for some unknown reason?!?!?!?
Change-Id: I7d9cdcc56263b27dfd4649dfb1da1d67761ee923
