[media] rtl2830.c: get rid of warning: no previous prototype

drivers/media/dvb-frontends/rtl2830.c:133:5: warning: no previous prototype for 'rtl2830_wr_reg_mask' [-Wmissing-prototypes]
drivers/media/dvb-frontends/rtl2830.c:153:5: warning: no previous prototype for 'rtl2830_rd_reg_mask' [-Wmissing-prototypes]
drivers/media/dvb-frontends/rtl2830.c:259:5: warning: no previous prototype for 'rtl2830_get_tune_settings' [-Wmissing-prototypes]

Cc: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>