Avoid that sparse reports the following warning on __fw_free_buf():

drivers/base/firmware_class.c:230:9: warning: context imbalance in '__fw_free_buf' - unexpected unlock
Signed-off-by: NBart Van Assche <bvanassche@acm.org>
Acked-by: NMing Lei <ming.lei@canonical.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>