spi/imx: prevent NULL pointer dereference in spi_imx_probe()

When no platform_data is present and either 'spi-num-chipselects' is
not defined in the DT or 'cs-gpios' has less entries than
'spi-num-chipselects' specifies, the NULL platform_data pointer is
being dereferenced.

Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>