spi: coldfire-qspi: Prevent NULL pointer dereference

If pdata->cs_control is NULL, we will hit NULL pointer dereference in
mcfqspi_cs_select() and mcfqspi_cs_deselect(). Thus add NULL test for
pdata->cs_control in probe().

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@linaro.org>