igbvf: use generic power management

Remove legacy PM callbacks and use generic operations. With legacy code,
drivers were responsible for handling PCI PM operations like
pci_save_state(). In generic code, all these are handled by PCI core.

The generic suspend() and resume() are called at the same point the legacy
ones were called. Thus, it does not affect the normal functioning of the
driver.

__maybe_unused attribute is used with .resume() but not with .suspend(), as
.suspend() is called by .shutdown().

Compile-tested only.

Signed-off-by: Vaibhav Gupta <vaibhavgupta40@gmail.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>