[media] cx24123/mb86a20s/s921: fix compiler warnings

In file included from drivers/media/common/b2c2/flexcop-fe-tuner.c:13:0:
drivers/media/dvb-frontends/cx24123.h:54:2: warning: 'cx24123_get_tuner_i2c_adapter' defined but not used [-Wunused-function]
  cx24123_get_tuner_i2c_adapter(struct dvb_frontend *fe)
  ^
In file included from drivers/media/usb/em28xx/em28xx-dvb.c:46:0:
drivers/media/dvb-frontends/s921.h:40:2: warning: 's921_get_tuner_i2c_adapter' defined but not used [-Wunused-function]
  s921_get_tuner_i2c_adapter(struct dvb_frontend *fe)
  ^
In file included from drivers/media/usb/em28xx/em28xx-dvb.c:55:0:
drivers/media/dvb-frontends/mb86a20s.h:49:2: warning: 'mb86a20s_get_tuner_i2c_adapter' defined but not used [-Wunused-function]
  mb86a20s_get_tuner_i2c_adapter(struct dvb_frontend *fe)
  ^
In file included from drivers/media/usb/cx231xx/cx231xx-dvb.c:35:0:
drivers/media/dvb-frontends/mb86a20s.h:49:2: warning: 'mb86a20s_get_tuner_i2c_adapter' defined but not used [-Wunused-function]
  mb86a20s_get_tuner_i2c_adapter(struct dvb_frontend *fe)
  ^

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>