[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>