test_firmware: fix missing unlock on error in config_num_requests_store()

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

Fixes: c92316bf ("test_firmware: add batched firmware tests")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>