Xilinx: SPI: Fix bits_per_word for transfers

The bits_per_word value can be set for each transfer, or can
be set to zero in each transfer in which case it should default
to the value in the driver.

The driver was fixed to properly check the bits_per_word in
the transfer that is passed in.

Signed-off-by: John Linn <john.linn@xilinx.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>