spi: spi-zynq-qspi: Fix stack violation bug

spi: spi-zynq-qspi: Fix stack violation bug

When the number of bytes for the op is greater than one, the read could
run off the end of the function stack and cause a crash.

This patch restores the behaviour of safely reading out of the original
opcode location.

Signed-off-by: Karen Dombroski <karen.dombroski@marsbioimaging.com>
Signed-off-by: Amit Kumar Mahapatra <amit.kumar-mahapatra@xilinx.com>
Link: https://lore.kernel.org/r/20210429053802.17650-3-amit.kumar-mahapatra@xilinx.com
Signed-off-by: Mark Brown <broonie@kernel.org>