mtd: rawnand: qcom: use FIELD_PREP and GENMASK

Use the bitfield macro FIELD_PREP, and GENMASK to
do the shift and mask in one go. This makes the code
more readable.

Reviewed-by: Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>
Signed-off-by: Md Sadre Alam <quic_mdalam@quicinc.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>