V4L/DVB (6553): tuner: replace default_mode_mask

The default_mode_mask global is replaced by a list of tuner structs. The
tuner driver now walks that list to see which radio and/or tv tuner
devices are registered to a given i2c adapter.

The default_mode_mask global had to go since this is no longer supported
with the new bus-based I2C API.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>