[media] xc2028: silence compiler warnings

There is now new tuner types which are not handled on that switch-case.
Print error if unknown tuner type is meet.

drivers/media/tuners/tuner-xc2028.c: In function ‘generic_set_freq’:
drivers/media/tuners/tuner-xc2028.c:1037:2: warning: enumeration value ‘V4L2_TUNER_ADC’ not handled in switch [-Wswitch]
  switch (new_type) {
  ^
drivers/media/tuners/tuner-xc2028.c:1037:2: warning: enumeration value ‘V4L2_TUNER_RF’ not handled in switch [-Wswitch]

Cc: Mauro Carvalho Chehab <m.chehab@samsung.com>
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>