mmc: Fix mmc_delay() function

Several fixes for mmc_delay():

 * Repair if-clause that was supposed to detect sub-hz delays.
 * Change yield() to cond_resched() as yield() no longer has the
   semantics we desire.
 * mmc_delay() is used to guarantee protocol delays, so we cannot
   return early (i.e. use _interruptable).

Based on patch by Amol Lad.

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