netdev: Change netdev_get_mtu() to return an error code.

netdev: Change netdev_get_mtu() to return an error code.

To make the netdev code more portable, it needs to support returning error
codes from functions that don't have them.  This commit changes
netdev_get_mtu() to return an error code and updates its caller.

(Currently netdev_get_mtu() won't ever return an error, but other future
implementations might.)