net: phy: c45: detect the BASE-T1 speed from the ability register

Read the ability to do 100BASE-T1 and 1000BASE-T1 from the extended
BASE-T1 ability register of the PHY.

Signed-off-by: Stefan Eichenberger <eichest@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>