spi: fsl_qspi: Preserve endianness of QSPI MCR

The endianness can be changed by RCW + PBI sequence. It may have
other than power on reset value.

Signed-off-by: York Sun <york.sun@nxp.com>
CC: Yuan Yao <yao.yuan@nxp.com>
CC: Peng Fan <peng.fan@nxp.com>
CC: Alison Wang <alison.wang@nxp.com>
Reviewed-by: Jagan Teki <jteki@openedev.com>