net: dsa: mv88e6xxx: add detection helper

Extract the common detection code which assigns the info structure to
the chip given the read switch ID.

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>