HAL_ChibiOS: limit RX timeout to 100ms

this prevents a very long timeout in begin(), replacement with a mutex
will happen in a separate PR

libraries/AP_HAL_ChibiOS/UARTDriver.cpp

@@ -877,6 +877,9 @@ void UARTDriver::write_pending_bytes_DMA(uint32_t n)
                         STM32_DMA_CR_MINC | STM32_DMA_CR_TCIE);
         dmaStreamEnable(txdma);
         uint32_t timeout_us = ((1000000UL * (tx_len+2) * 10) / _baudrate) + 500;
+        // prevent very long timeouts at low baudrates which could cause another thread
+        // using begin() to block
+        timeout_us = MIN(timeout_us, 100000UL);
         chSysUnlock();
         // wait for the completion or timeout handlers to signal that we are done
         eventmask_t mask = chEvtWaitAnyTimeout(EVT_TRANSMIT_DMA_COMPLETE, chTimeUS2I(timeout_us));