net: phy: move phy MMD accessors to phy-core.c

Move the phy_(read|write)__mmd() helpers out of line, they will become
our main MMD accessor functions, and so will be a little more complex.
This complexity doesn't belong in an inline function.  Also move the
_indirect variants as well to keep like functionality together.

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>