media: saa7146: fix array overflow in vidioc_s_audio()

[ Upstream commit 8e4d86e241cf035d6d3467cd346e7ce490681937 ]

The "a->index" value comes from the user via the ioctl.  The problem is
that the shift can wrap resulting in setting "mxb->cur_audinput" to an
invalid value, which later results in an array overflow.

Fixes: 66804277 ("[media] mxb: fix audio handling")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>