net: dsa: split dsa_switch_setup into two functions

Split the part of dsa_switch_setup() which is responsible for allocating
and initializing a 'struct dsa_switch' and the part which is doing a
given switch device setup and slave network device creation.

This is a preliminary change to allow a separate caller of
dsa_switch_setup_one() which may have externally initialized the
dsa_switch structure, outside of dsa_switch_setup().

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