[media] Fix typos in VIDEO_CX231XX_DVB Kconfig entry

Commit ede676c7 ("[...] add digital support for PV SBTVD hybrid")
added "select MEDIA_TUNER_NXP18271" to the VIDEO_CX231XX_DVB Kconfig
entry. But there's no Kconfig symbol MEDIA_TUNER_NXP18271. That should
have been MEDIA_TUNER_TDA18271. (The code added in that commit uses
tda18271_attach, which is only available if MEDIA_TUNER_TDA18271 is
set.)

The selects of MEDIA_TUNER_XC5000 and MEDIA_TUNER_TDA18271 should only
be done if MEDIA_TUNER_CUSTOMISE isn't set.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>