mgb: Fix nop admin interrupt handling

Previously mgb_admin_intr printed a diagnostic message if no interrupt
status bits were set, but it's not valid to call device_printf() from a
filter. Just drop the message as it has no user-facing value.

Also return FILTER_STRAY in this case - there is nothing further for
the driver to do.

Reviewed by: kbowling
MFC after: 1 week
Fixes: 8890ab7758b8 ("Introduce if_mgb driver...")
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D32231