spi/bitbang: Drop empty setup() functions

Now that the bitbang core does not require a setup() function we can
drop the check in the altera, nuc900 and xilinx drivers.

Signed-off-by: Mark Brown <broonie@linaro.org>

diff --cc drivers/spi/spi-altera.c
Simple merge

diff --cc drivers/spi/spi-nuc900.c
Simple merge

diff --cc drivers/spi/spi-xilinx.c
Simple merge