spi: spi-qpic-snand: use CW_PER_PAGE_MASK bitmask

spi: spi-qpic-snand: use CW_PER_PAGE_MASK bitmask

Change the code to use the already defined CW_PER_PAGE_MASK
bitmask along with the FIELD_PREP() macro instead of using
magic values.

This makes the code more readable. It also syncs the affected
codes with their counterparts in the 'qcom_nandc' driver, so it
makes it easier to spot the differences between the two
implementations.

No functional changes intended.

Signed-off-by: Gabor Juhos <j4g8y7@gmail.com>
Reviewed-by: Md Sadre Alam <quic_mdalam@quicinc.com>
Link: https://patch.msgid.link/20250515-qpic-snand-use-bitmasks-v1-1-11729aeae73b@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>