net: phy: mxl-gpy: rename MMD_VEND1 macros to match datasheet

Rename the temperature sensors macros to match the names in the
datasheet.

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