net: dsa: qca8k: move qca8k_port_to_phy() to header

Move qca8k_port_to_phy() to qca8k header as it's useful for future
reference in Switch LEDs module since the same logic is applied to get
the right index of the switch port.
Make it inline as it's simple function that just decrease the port.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Michal Kubiak <michal.kubiak@intel.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>