spi/dw_spi: clean the cs_control code

commit 052dc7c45i "spi/dw_spi: conditional transfer mode change"
introduced cs_control code, which has a bug by using bit offset
for spi mode to set transfer mode in control register. Also it
forces devices who don't need cs_control to re-configure the
control registers for each spi transfer. This patch will fix them

Signed-off-by: Feng Tang <feng.tang@intel.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>