spi: pxa2xx: Save other reg_cs_ctrl bits when configuring chip select

Upcoming Intel platforms use LPSS SPI_CS_CONTROL register bits 15:12 for
configuring the chip select polarities. Touch only chip select SW mode and
state bits when enabling the software chip select control in order to not
clear any other bits in the register.

Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>