[media] dib8000: warning fix: declare internal functions as static

drivers/media/dvb-frontends/dib8000.c:2412:5: warning: no previous prototype for 'dib8000_wait_lock' [-Wmissing-prototypes]
drivers/media/dvb-frontends/dib8000.c:2688:5: warning: no previous prototype for 'dib8000_get_symbol_duration' [-Wmissing-prototypes]

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