Platform/RaspberryPi: fix MmcTransferBlock (writes) regression for eMMC

Way back when I added write block validation [0], I had screwed the
logic up, breaking eMMC.

This starts to matter on the CM3, which has eMMC.

[0] andreiw/RaspberryPiPkg@bdea35dc3f1652bf

Signed-off-by: Andrei Warkentin <andrey.warkentin@gmail.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@arm.com>

Platform/RaspberryPi/Drivers/MmcDxe/MmcBlockIo.c

@@ -47,6 +47,7 @@ ValidateWrittenBlockCount (
     /*
      * Not on MMC.
      */
+    *TransferredBlocks = Count;
     return EFI_SUCCESS;
   }