fix(mpm): update internal module (#2314)

* Internal MPM update: UART context must be removed last

This fixes the crash / reboot observed in issue #2310

* Fix DMA fifo clear() method

radio/src/dmafifo.h

@@ -36,7 +36,7 @@ class DMAFifo
 
     void clear()
     {
-      ridx = 0;
+      ridx = N - stream->NDTR;
     }
 
     uint32_t size()

radio/src/io/multi_firmware_update.cpp

@@ -122,8 +122,8 @@ class MultiInternalUpdateDriver: public MultiFirmwareUpdateDriver
     void deinit(bool inverted) override
     {
       IntmoduleSerialDriver.deinit(uart_ctx);
-      uart_ctx = nullptr;
       clear();
+      uart_ctx = nullptr;
     }
 };
 #endif