spi: omap2-mcspi: Remove the macro MOD_REG_BIT

Remove the macro MOD_REG_BIT instead make the bit field modifications
directly. This deletes a branch operation in cases where the the set
is predecided. While at it optimise two sequential bit clear in one step.

Acked-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Shubhrajyoti D <shubhrajyoti@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>