imxmmc: fix crash when no platform data is provided

Don't crash if no platform data is provided.
In this case assume that card is present.

Signed-off-by: Paulius Zaleckas <paulius.zaleckas@teltonika.lt>
Acked-by: Pavel Pisa <pisa@cmp.felk.cvut.cz>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>