net: dsa: mv88e6xxx: read switch ID in probe

Read the switch ID only once, at probe time, to avoid multiple read
accesses and MII bus checking.

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