spi/xilinx: Revert master->setup function removal

master->setup() must be initialized to be able
to successfully run spi_bitbang_start()
and satisfy if/else logic there.

"spi: convert drivers to use bits_per_word_mask"
(sha1: 24778be2

)

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Mark Brown <broonie@linaro.org>