net: dsa: mv88e6xxx: add switch register helpers

The mixed assignments, allocations and registrations in the probe code
make it hard to follow the logic and figure out what is DSA or chip
specific.

Extract the struct dsa_switch related code in a simple
mv88e6xxx_register_switch helper function.

For symmetry in the code, add a mv88e6xxx_unregister_switch function.

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>