spi: omap2-mcspi: Fix GPIO chip select support

The OMAP2_MCSPI_CHCONF_FORCE must be toggled even when using GPIO
chip selects. This patch conditionally calls the omap2_mcspi_set_cs
function to do so when using GPIO chip selects.

Signed-off-by: Michael Welling <mwelling@ieee.org>
Signed-off-by: Mark Brown <broonie@kernel.org>