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] https://github.com/andreiw/RaspberryPiPkg/commit/bdea35dc3f1652bf
Signed-off-by: Andrei Warkentin <andrey.warkentin@gmail.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@arm.com>