net: mdio: add helpers to extract clause 45 regad and devad fields

[ Upstream commit c6af53f0 ]

Add a couple of helpers and definitions to extract the clause 45 regad
and devad fields from the regnum passed into MDIO drivers.

Tested-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Stable-dep-of: 88513469 ("net: stmmac: Assign configured channel value to EXTTS event")
Signed-off-by: Sasha Levin <sashal@kernel.org>