spi/omap_mcspi: Fix broken last word xfer

Commit adef658 "spi/omap_mcspi: catch xfers of non-multiple SPI word size"
broke the transmission of last word in cases where access is multiple of
word size and word size is 16 or 32 bits.

Fix this by replacing the test "c > (word_len>>3)" in do-while loops with
"c >= 'pointer increment size'". This ensures that the last word is
transmitted in above case and still allow to break the loop and prevent
variable c underflow in cases where word size != 'pointer increment size'.

Signed-off-by: Jarkko Nikula <jhnikula@gmail.com>
Tested-by: Sourav Poddar<sourav.poddar@ti.com>
Acked-by: Michael Jones <michael.jones@matrix-vision.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>