Platform/RPi/MmcDxe: Improve MMC driver stability

For reasons that aren't entirely understood, SCR reads can randomly
fail on the Raspberry Pi 4. It can be on the first boot or during
subsequent reboots.

To alleviate this, and improve the overall behavior of the code,
we modify CheckCardsCallback () to retry InitializeMmcDevice ()
in case of failure.

Signed-off-by: Pete Batard <pete@akeo.ie>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>