spi: spi-mxs: Remove mxs_spi_enable and mxs_spi_disable

These functions consist of nothing but one single writel call and are
only called once.  And the names really aren't accurate or clear,
since they don't enable or disble SPI.  Rather they set the bit that
controls the state of CS at the end of transfer.  It easier to follow
the code to just set this bit with a writel() along with all the other
bits being set in the same function.

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>