rtl818x_pci: use generic power management

rtl818x_pci: use generic power management

Earlier, drivers had to manage the device's power states, and related
operations, themselves. With the generic approach, these are done by PCI
core.

The only driver-specific jobs, .suspend() and .resume() doing were invoking
PCI helper functions pci_save/restore_state() and
pci_set_power_state(). This is not recommeneded as PCI core takes care of
that. Hence they became empty-body functions, thus define them NULL.

Compile-tested only.

Signed-off-by: Vaibhav Gupta <vaibhavgupta40@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200623094454.12427-1-vaibhavgupta40@gmail.com