spi: fsl_qspi: Use GENMASK

Replace numeric mask hexcodes with GENMASK macro
in fsl_qspi

Cc: York Sun <yorksun@freescale.com>
Cc: Haikun Wang <Haikun.Wang@freescale.com>
Signed-off-by: Jagan Teki <jteki@openedev.com>