net: dsa: mv88e6xxx: add chip allocation helper

Add an helper function to allocate the chip structure at the beginning
of the probe functions. It will be used to initialize the SMI access.

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>