firmware loader: Add sparse annotation

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: Bart Van Assche <bvanassche@acm.org>
Acked-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/base/firmware_class.c

@@ -228,6 +228,7 @@ static int fw_lookup_and_allocate_buf(const char *fw_name,
 }
 
 static void __fw_free_buf(struct kref *ref)
+	__releases(&fwc->lock)
 {
 	struct firmware_buf *buf = to_fwbuf(ref);
 	struct firmware_cache *fwc = buf->fwc;