media: ngene: use defines to identify the demod_type

Make it more clear which demod_type is used for which hardware by having
defines for the possible demod_type values. With that, change the
demod_type evaluation in tuner_attach_probe() to a switch-case instead
of an if() for each possible value.

Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>