net: dsa: mv88e6xxx: Fix typ0 when configuring 2.5Gbps

In order to enable 2.5Gbps mode, we need the base speed of 10G, plus
the Alt bit setting. Fix a typ0 that used 1Gb base speed.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>