net: stmmac: make in-band AN mode parsing is supported for non-DT

Not all platform uses DT, so phylink_parse_mode() will skip in-band setup
of pl->supported and pl->link_config.advertising entirely. So, we add the
setting of ovr_an_inband flag to make it works for non-DT platform.

Signed-off-by: Ong Boon Leong <boon.leong.ong@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>