spi: omap2-mcspi: add runtime PM to set_cs()

Since commit ddcad7e9 omap2_mcspi_set_cs() is called without
runtime power management requested.  This patch fixes the problem by
requesting runtime power management in omap2_mcspi_set_cs().

Reported-By: Pali Rohár <pali.rohar@gmail.com>
Fixes: ddcad7e9 (spi: omap2-mcspi: Fix native cs with new set_cs)
Tested-By: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Sebastian Reichel <sre@kernel.org>
Acked-by: Michael Welling <mwelling@ieee.org>
Signed-off-by: Mark Brown <broonie@kernel.org>