test_firmware: make local symbol test_fw_config static

Fixes the following sparse warnings:

lib/test_firmware.c:99:20: warning:
 symbol 'test_fw_config' was not declared. Should it be static?

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