mmc: increase SD write timeout for crappy cards

It seems that some cards are slightly out of spec and occasionally
will not be able to complete a write in the alloted 250 ms [1].
Incease the timeout slightly to allow even these cards to function
properly.

[1] http://lkml.org/lkml/2008/9/23/390

Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>