test_firmware: fix missing unlock on error in config_num_requests_store()

commit a5e1923356505e46476c2fb518559b7a4d9d25b1 upstream.

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")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>