net: dsa: Mop up remaining NET_DSA_HWMON references

Previous patches have moved the temperature sensor code into the
Marvell PHYs. A few now dead references to NET_DSA_HWMON were left
behind. Go reap them.

Reported-by: Valentin Rothberg <valentinrothberg@gmail.com>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>