realtek/8139cp: use generic power management

compile-tested only

With legacy PM hooks, it was the responsibility
of a driver to manage PCI states and also
device's power state. The generic approach is
to let PCI core handle the work.

The suspend callback enables/disables PCI wake
on the basis of "cp->wol_enabled" variable
which is unknown to PCI core. To utilise its
need, call device_set_wakeup_enable().

Signed-off-by: Vaibhav Gupta <vaibhavgupta40@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>