spi: spi-mxs: Remove unused bits_per_word variable

The bits_per_word variable is not used after commit 24778be2
"spi: convert drivers to use bits_per_word_mask".

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Marek Vasut <marex@denx.de>
Signed-off-by: Mark Brown <broonie@linaro.org>