net: dsa: Add helper function to obtain PHY device of a given port

In preparation for having more call sites attempting to obtain a
reference against a PHY device corresponding to a particular port,
introduce a helper function for that purpose.

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