V4L/DVB (11078): au0828: properly handle non-existent analog inputs

It is not valid to look for dev->board.input == NULL to detect an undefined
analog configuration section, since it is a member of the struct and not a
pointer (hence it will *always* be non-NULL).  Do the check based on whether
the first input is actually a valid input type instead.

Thanks to Michael Krufky <mkrufky@linuxtv.org> for providing sample hardware
of various configurations to test with.

Signed-off-by: Devin Heitmueller <dheitmueller@linuxtv.org>
Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>