V4L/DVB (5806): Bttv: Be consistent in using symbolic names instead of constants

For svhs, tuner, and tuner_type, be consistent in using UNSET instead of -1.

For tuner_type also consistently use the existing constants:
0  => TUNER_TEMIC_PAL
1  => TUNER_PHILIPS_PAL_I
2  => TUNER_PHILIPS_NTSC
4  => TUNER_ABSENT
5  => TUNER_PHILIPS_PAL
21 => TUNER_TEMIC_4039FR5_NTSC
25 => TUNER_LG_PAL_I_FM

Signed-off-by: Trent Piepho <xyzzy@speakeasy.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>