spi: imx: Take in account bits per word instead of assuming 8-bits

The IMX spi driver has a hardcoded 8, breaking the driver for word
lengths other than 8.

Signed-off-by: Stefan Moring <stefanmoring@gmail.com>
Reported-by: Sebastian Reichel <sre@kernel.org>
Fixes: 15a6af94

 ("spi: Increase imx51 ecspi burst length based on transfer length")
Tested-by: Sebastian Reichel <sre@kernel.org>
Link: https://lore.kernel.org/r/20230917164037.29284-1-stefanmoring@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>