net: phy: make phy_(read|write)_mmd() generic MMD accessors

Make phy_(read|write)_mmd() generic 802.3 clause 45 register accessors
for both Clause 22 and Clause 45 PHYs, using either the direct register
reading for Clause 45, or the indirect method for Clause 22 PHYs.
Allow this behaviour to be overriden by PHY drivers where necessary.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>