Add genphy_c45_config_aneg() function to phy-c45.c

Commit 34786005 ("net: phy: prevent PHYs w/o Clause 22 regs from calling
genphy_config_aneg") introduced a check that aborts phy_config_aneg()
if the phy is a C45 phy.
This causes phy_state_machine() to call phy_error() so that the phy
ends up in PHY_HALTED state.

Instead of returning -EOPNOTSUPP, call genphy_c45_config_aneg()
(analogous to the C22 case) so that the state machine can run
correctly.

genphy_c45_config_aneg() closely resembles mv3310_config_aneg()
in drivers/net/phy/marvell10g.c, excluding vendor specific
configurations for 1000BaseT.

Fixes: 22b56e82 ("net: phy: replace genphy_10g_driver with genphy_c45_driver")

Signed-off-by: Marco Hartmann <marco.hartmann@nxp.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>