net: phy: mxl-gpy: cache PHY firmware version

Cache the firmware version during probe. There is no need to read the
firmware version on every autonegotiation.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>