net: systemport: Create helper to set MPD

Create a helper function to turn on/off MPD, this will be used to avoid
duplicating code as we are going to add additional types of wake-up
types.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>