spi/omap: fix D0/D1 direction confusion

0384e90b ("spi/mcspi: allow configuration of pin directions") did what
it claimed to do the wrong way around. D0/D1 is configured as output by
*clearing* the bits in the conf registers, hence also breaking the
former default behaviour.

Fix this before that change is merged to mainline.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>