V4L/DVB (10178): dvb_frontend: Fix some sparse warnings due to static symbols

/home/v4l/master/v4l/dvb_frontend.c:838:19: warning: symbol 'dtv_cmds' was not declared. Should it be static?
/home/v4l/master/v4l/dvb_frontend.c:1035:6: warning: symbol 'dtv_property_dump' was not declared. Should it be static?
/home/v4l/master/v4l/dvb_frontend.c:1066:5: warning: symbol 'is_legacy_delivery_system' was not declared. Should it be static?
/home/v4l/master/v4l/dvb_frontend.c:1080:6: warning: symbol 'dtv_property_cache_sync' was not declared. Should it be static?
/home/v4l/master/v4l/dvb_frontend.c:1132:6: warning: symbol 'dtv_property_legacy_params_sync' was not declared. Should it be static?
/home/v4l/master/v4l/dvb_frontend.c:1187:6: warning: symbol 'dtv_property_adv_params_sync' was not declared. Should it be static?
/home/v4l/master/v4l/dvb_frontend.c:1222:6: warning: symbol 'dtv_property_cache_submit' was not declared. Should it be static?
/home/v4l/master/v4l/dvb_frontend.c:1253:5: warning: symbol 'dtv_property_process_get' was not declared. Should it be static?
/home/v4l/master/v4l/dvb_frontend.c:1362:5: warning: symbol 'dtv_property_process_set' was not declared. Should it be static?

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