[media] mb86a20s: fix ISDB-T mode handling

The driver was reporting an incorrect mode, when mode 2
is selected.

While testing it, noticed that neither mode 1 or guard
interval 1/32 is supported by this device. Document it,
and ensure that it will report _AUTO when it doesn't lock,
in order to not report a wrong detection to userspace.

Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>