spi: omap2-mcspi: Fix native cs with new set_cs

GPIO chip select patch series appears to have broken the native chip select
support. This patch pulls the manual native chip select toggling out of
the transfer_one routine and adds a set_cs routine.

Tested natively on AM3354 with SPI serial flash on spi0cs0.

Reported-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Michael Welling <mwelling@ieee.org>
Tested-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Mark Brown <broonie@kernel.org>