net: dsa: move PHY setup on DSA MII bus to its own function

In preparation for dealing with indirect reads and writes towards
certain PHY devices, move the code which deals with binding the PHY
device to the slave MII bus created by DSA to its own function:
dsa_slave_phy_connect().

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