firmware/sniffer: Disable TIMEOUT interrupts in USART IER on exit

Not critical (we disable the USART interrupts in NVIC anyway), but
if Sniffer_init() enables this flag, it's good style for Sniffer_exit()
to disable it.

Change-Id: I92e16a160d60fcab33c81e0cf074088b9f20b9ae

firmware/libcommon/source/sniffer.c

@@ -913,7 +913,7 @@ void Sniffer_exit(void)
 {
  TRACE_INFO("Sniffer exit\n\r");
  /* Disable USART */
- USART_DisableIt(sniff_usart.base, US_IER_RXRDY);
+ USART_DisableIt(sniff_usart.base, US_IER_RXRDY | US_IER_TIMEOUT);
  /* NOTE: don't forget to set the IRQ according to the USART peripheral used */
  NVIC_DisableIRQ(IRQ_USART_SIM);
  USART_SetReceiverEnabled(sniff_usart.base, 0);