ofp-util: return BAD_TYPE for consistency at ofputil_decode_group_mod().

ofp-util: return BAD_TYPE for consistency at ofputil_decode_group_mod().

Code earlier in this function validated that gm->type is valid, so
the default case at the end of the function would be impossible, but
since the code can change or perhaps get copied, return the correct
error to keep consistency and avoid reintroducing the bug fixed by
commit 09d4b951636b ("ofp-util: Reject bad group type and command
with error instead of abort.")

Signed-off-by: Flavio Leitner <fbl@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>