spi: spi-zynqmp-gqspi: fix use-after-free in zynqmp_qspi_exec_op

In the Linux kernel, the following vulnerability has been resolved:

spi: spi-zynqmp-gqspi: fix use-after-free in zynqmp_qspi_exec_op

When handling op->addr, it is using the buffer "tmpbuf" which has been freed. This will trigger a use-after-free KASAN warning. Let's use temporary variables to store op->addr.val and op->cmd.opcode to fix this issue.