media: allegro: read bitrate mode directly from control

There is no need to copy the bitrate mode to a field in the channel and
the value can be read directly from the control.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>