net: phy: add helpers for comparing phy IDs

There are several places which open code comparing PHY IDs. Provide a
couple of helpers to assist with this, using a slightly simpler test
than the original:

- phy_id_compare() compares two arbitary PHY IDs and a mask of the
  significant bits in the ID.
- phydev_id_compare() compares the bound phydev with the specified
  PHY ID, using the bound driver's mask.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>