[PPPoX/E]: return ENOTTY on unknown ioctl requests

here another patch for the PPPoX/E code that makes sure that ENOTTY is
returned for unknown ioctl requests rather than 0 (and removes another
unneeded initializer which I didn't bother creating a separate patch for).

Signed-off-by: Florian Zumbiehl <florz@florz.de>
Signed-off-by: David S. Miller <davem@davemloft.net>