spi/omap_mcspi: catch xfers of non-multiple SPI word size

If an SPI access was not a multiple of the SPI word size,
the while() loop would spin and the rx/tx ptrs would be incremented
indefinitely.

Signed-off-by: Michael Jones <michael.jones@matrix-vision.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>