adm8211: use generic power management

With legacy PM, drivers themselves were responsible for managing the
device's power states and takes care of register states.

After upgrading to the generic structure, PCI core will take care of
required tasks and drivers should do only device-specific operations.

In the case of adm8211, after removing PCI helper functions, .suspend()
and .resume() became empty-body functions. Hence, define them NULL and
use dev_pm_ops.

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/20200629035031.169670-1-vaibhavgupta40@gmail.com