net: phy: mxl-gpy: rename the FW type field name

Align the firmware field name with the reference manual where it is
called "major".

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