V4L/DVB (10192): em28xx: fix input selection

em28xx were trying to access the third input entry, even for boards that
don't support it.

This patch reviews the input mux selection fixing this bug and a few
other troubles, like not validating the input on one userspace ioctl.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>