net: dsa: mv88e6xxx: add SMI init helper

Add an helper function to isolate SMI specific assignments and checks.

This function will later help choosing the different SMI accesses based
of the compatible info.

Since the chip structure is already allocated in the legacy probe, use
the mv88e6xxx_reg_read access routine instead of __mv88e6xxx_reg_read.

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>