spi: xilinx: Convert to let spi core validate bits_per_word

Set bits_per_word_mask so spi core will reject transfers that attempt to use
an unsupported bits_per_word value.

spi_bitbang requires custom setup_transfer() to be defined if there is a
custom txrx_bufs(). Thus keep the empty xilinx_spi_setup_transfer() function
in the code.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Mark Brown <broonie@linaro.org>