mmc_spi: wait more bytes for card response

Some cards are slower than the standard allows and need more
time to respond to a command. Max. observed number of bytes
was 12.

Signed-off-by: Wolfgang Muees <wolfgang.mues@auerswald.de>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>