mmc: fix card busy polling

A MMC/SD card may always go into the programming state (and hence be
busy) after a block write. Therefore always check the card state, even
after single block writes. On the other hand there is no need to check
the card status after a read.

Also make sure that errors during busy polling are propagated upwards.

Signed-off-by: Jan Kloetzke <jan.kloetzke@dspg.com>
Cc: Andy Fleming <afleming@gmail.com>