media: saa7164: remove redundant assignment to variable idx

Variable idx is being assigned a value that is never read, it is being
re-assigned a new value in the following for-loop. The assignment is
redundant and can be removed.

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>