[media] ts2020: fix two warnings added by changeset 73f0af44

drivers/media/dvb-frontends/ts2020.c: In function 'ts2020_set_params':
drivers/media/dvb-frontends/ts2020.c:126:47: warning: variable 'div4' set but not used [-Wunused-but-set-variable]
drivers/media/dvb-frontends/ts2020.c: At top level:
drivers/media/dvb-frontends/ts2020.c:262:5: warning: no previous prototype for 'ts2020_get_signal_strength' [-Wmissing-prototypes]

Cc: Konstantin Dimitrov <kosio.dimitrov@gmail.com>
Cc: Igor M. Liplianin <liplianin@me.by>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>