net: phy: corrected the return value for genphy_check_and_restart_aneg and genphy_c45_check_and_restart_aneg

When auto-negotiation is not required, return value should be zero.

Changes v1->v2:
- improved comments and code as Andrew Lunn and Heiner Kallweit suggestion
- fixed issue in genphy_c45_check_and_restart_aneg as Russell King
  suggestion.

Fixes: 2a10ab04 ("net: phy: add genphy_check_and_restart_aneg()")
Fixes: 1af9f168 ("net: phy: add genphy_c45_check_and_restart_aneg()")
Signed-off-by: Sudheesh Mavila <sudheesh.mavila@amd.com>
Reviewed-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>