[media] cx25821: off by one in cx25821_vidioc_s_input()

If "i" is 2 then when we call cx25821_video_mux() we'd end up going
past the end of the cx25821_boards[dev->board]->input[].

The INPUT() macro obfuscates what's going on in that function so it's
a bit hard to follow.  And as Mauro points out the hard coded 2 is
not very helpful.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>