mmc_spi: allow higher timeouts for SPI mode

Some SD cards have very high timeouts in SPI mode.
So adjust the timeouts from theory to practice.

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>