V4L/DVB (7060): em28xx: remove has_tuner

has_tuner flag doesn't make much sense, since tuner_type=TUNER_ABSENT
means the same thing.

Having two ways to say that a tuner is not present is
not nice, since it may lead to bad setups. In fact, with the previous
code, if a device were using has_tuner=0, but the user forces a tuner,
with modprobe option tuner=type, the modprobe option won't work.

Also, tveeprom returns TUNER_ABSENT, when tuner is unknown or absent.
So, with the previous logic, in this case, the driver should set
has_tuner=0, or has_tuner=1 otherwise.

Instead of adding several additional tests and setups, better just to
remove .has_tuner.

Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>