media: em28xx: fix handler for vidioc_s_input()

The a->index is not the name of the internal amux entry,
but, instead a value from zero to the maximum number
of audio inputs.

As the actual available inputs depend on each board, build
it dynamically.

This is broken for a really long time. On a quick check,
since at least commit 195a4ef6 ("V4L/DVB (6585): Convert
em28xx to video_ioctl2") this was not implemented right.

Fixes: 195a4ef6 ("V4L/DVB (6585): Convert em28xx to video_ioctl2")

Cc: stable@vger.kernel.org
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>