spi/spi_nuc900: Use spi_bitbang_stop instead of spi_unregister_master in nuc900_spi_remove

Calling spi_bitbang_stop() will also destroy bitbang->workqueue,
which is created by calling spi_bitbang_start() in nuc900_spi_probe().

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>