ar71xx: allow to override at803x sgmii aneg status

ar71xx: allow to override at803x sgmii aneg status

When checking the outcome of the PHY autonegotiation status, at803x
currently returns false in case the SGMII side is not established.

Due to a hardware-bug, ag71xx needs to fixup the SoCs SGMII side, which
it can't as it is not aware of the link-establishment.

This commit allows to ignore the SGMII side autonegotiation status to
allow ag71xx to do the fixup work.

Signed-off-by: David Bauer <mail@david-bauer.net>
(cherry picked from commit 4e39e213af7e3e0cd747403e8c227e145cfef988)