V4L/DVB (3402): Fix handling of VIDIOC_G_TUNER audmode in msp3400

- Fix handling of VIDIOC_G_TUNER audmode in msp3400: audmode
is only changed by the user with S_TUNER, never by the driver.

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