net: dsa: extract dsa switch tree setup and removal

Extract the core logic that setups a 'struct dsa_switch_tree' and
removes it, update dsa_probe() and dsa_remove() to use the two helper
functions. This will be useful to allow for other callers to setup
this structure differently.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>