s32gen1: qspi: use get_cmd_opcode to get valid opcode.

Because of upstream commit 0e30f472

 ("mtd: spi-nor: add support
for DTR protocol"), the 8-8-8 DTR mode read opcode needs to be adjusted
to be "(code << 8) | code" or (code << 8 | ~ code) according to the cmd
opcode extension which is clarified as "repeat" or "revert".

Since the s32g qspi driver only recognize 8-bit cmd opcode, we need get
correct 8-8-8 read opcode from op->cmd.opcode by adding new function
get_cmd_opcode.

Signed-off-by: Quanyang Wang <quanyang.wang@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>