Add the missing unlock before return from function zynqmp_qspi_exec_op()
in the error handling case.

Fixes: a0f65be6 ("spi: spi-zynqmp-gqspi: add mutex locking for exec_op")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Link: https://lore.kernel.org/r/20210412160025.194171-1-weiyongjun1@huawei.comSigned-off-by: Mark Brown <broonie@kernel.org>