Add helper to read and write expansion registers without taking the mdio
lock.

Please note, that this changes the semantics of the read and write.
Before there was no lock between selecting the expansion register and
the actual read/write. This may lead to access failures if there are
parallel accesses. Instead take the bus lock during the whole access
cycle.
Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>