[media] stk1160: make some functions static

As warned by gcc:

drivers/media/usb/stk1160/stk1160-ac97.c:117:5: warning: no previous prototype for 'stk1160_has_audio' [-Wmissing-prototypes]
 int stk1160_has_audio(struct stk1160 *dev)
     ^~~~~~~~~~~~~~~~~
drivers/media/usb/stk1160/stk1160-ac97.c:125:5: warning: no previous prototype for 'stk1160_has_ac97' [-Wmissing-prototypes]
 int stk1160_has_ac97(struct stk1160 *dev)
     ^~~~~~~~~~~~~~~~

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>