V4L/DVB (9617): tvtime: remove generic_checkmode callback

generic_checkmode() were called, via a callback, for some tvaudio chips.
There's just one callback code used on all those boards. So, it makes no
sense on keeping this as a callback.

Since there were some OOPS reported on tvaudio on kerneloops.org, this
patch removes this callback, adding the code at the only place were it
is called: inside chip_tread. A flag were added to indicate the need for
a kernel thread to set stereo mode on cards that needs it.

Using this more direct approach simplifies the code, making it more
robust against human errors.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>