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>