spi: spi-mxs: Remove full duplex check, spi core already does it

spi: spi-mxs: Remove full duplex check, spi core already does it

Because the driver sets the SPI_MASTER_HALF_DUPLEX flag, the spi core
will check transfers to insure they are not full duplex.  It's not
necessary to check that in the spi-mxs driver as well.

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>