[media] stb0899_drv: get rid of warning: no previous prototype

drivers/media/dvb-frontends/stb0899_drv.c:1263:5: warning: no previous prototype for 'stb0899_get_dev_id' [-Wmissing-prototypes]

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