[media] media: au0828 enable the right media source when input changes

Change vidioc_s_input() to enable the media source for the newly
selected input.

v4l2-core enables source before calling au0828's vidioc_s_input()
handler. Hence, when input selection changes, media source for the
newly selected input needs to be enabled.

Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>