mtd: rawnand: qcom: finish converting register to FIELD_PREP

With some research in some obscure old QSDK, it was possible to find the
MASK of the last register there were still set with raw shift and
convert them to FIELD_PREP API.

This is only a cleanup and modernize the code a bit and doesn't make
any behaviour change.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>