spi: mxc_spi: Fix spi clock glitch durant reset

Measuring the spi clock line on a scope shows a 'glitch' during the reset of the
spi.

Fix this by toggling only the MXC_CSPICTRL_EN bit, so that the clock line becomes
always stable.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Stefano Babic <sbabic@denx.de>