liquidio: use generic power management

Drivers should not use legacy power management as they have to manage power
states and related operations, for the device, themselves. This driver was
handling them with the help of PCI helper functions.

With generic PM, all essentials will be handled by the PCI core. Driver
needs to do only device-specific operations.

The driver defined empty-body .suspend() and .resume() callbacks earlier.
They can now be define NULL and bind with "struct dev_pm_ops" variable.

Compile-tested only.

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