spi: imx: rename 'bpw' variables

'bpw' is ambiguous and only the context makes sure if bytes_per_word
or bits_per_word is meant. Use the full names instead to make reading
the code easier.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Mark Brown <broonie@kernel.org>