mmc_spi: allow setting of spi mode 3

Allow the platform data structures to specify spi mode 3
(if there is a pullup on the clock line or the spi hardware
is not able to serve spi mode 0).

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>