spi: spi-mxs: Remove check of spi mode bits

The spi core already checks for a slave setting mode bits that we
didn't list as supported when the master was registered.  There is no
need to do it again in the master driver.

Signed-off-by: Trent Piepho <tpiepho@gmail.com>
Cc: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Mark Brown <broonie@linaro.org>