[media] fmdrv_common: fix sparse warning

drivers/media/radio/wl128x/fmdrv_common.c:178:6: warning: symbol 'g_st_write' was not declared. Should it be static?
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>

drivers/media/radio/wl128x/fmdrv_common.c

@@ -175,7 +175,7 @@ static int_handler_prototype int_handler_table[] = {
 	fm_irq_handle_intmsk_cmd_resp
 };
 
-long (*g_st_write) (struct sk_buff *skb);
+static long (*g_st_write) (struct sk_buff *skb);
 static struct completion wait_for_fmdrv_reg_comp;
 
 static inline void fm_irq_call(struct fmdev *fmdev)