usbnet: pegasus: set wakeup enable in set_wol

This patch calls device_set_wakeup_enable() inside set_wol
callback, so that turning on WOL from user mode utility
can make the 'wakeup' of pegasus device to be enabled, then
remote wakeup may be enabled before putting into sleep.

Cc: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Cc: Petko Manolov <petkan@users.sourceforge.net>
Signed-off-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>