[media] rtl2832: get rid of warning: no previous prototype

drivers/media/dvb-frontends/rtl2832.c:268:5: warning: no previous prototype for 'rtl2832_rd_demod_reg' [-Wmissing-prototypes]
drivers/media/dvb-frontends/rtl2832.c:308:5: warning: no previous prototype for 'rtl2832_wr_demod_reg' [-Wmissing-prototypes]
drivers/media/dvb-frontends/rtl2832.c:513:5: warning: no previous prototype for 'rtl2832_get_tune_settings' [-Wmissing-prototypes]

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