spi: spi-mxs: Clean up setup_transfer function

spi: spi-mxs: Clean up setup_transfer function

It can't be called with a NULL transfer anymore so it can be simplified
to not check for that.

Fix indention of line-wrapped code to Linux standard.

The transfer pointer can be const.

It's not necessary to check if the spi_transfer's speed_hz is zero, as
the spi core also fills it in from the spi_device.  However, the spi
core does not check if spi_device's speed is zero so we have to do
that still.

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>