net: Add preliminaly support for suspending/resuming a net interface

Such state needs to be set _from_ the PM API functions and not the other
way round. So if a network device driver does not support such API, it
will not be able to set the core net_if on PM state, obviously.

Currently, these functions only set/unset NET_IF_SUSPENDED flag.

More logic will be added later, to decide whether the net_if can be
actually set to suspend mode or not and also to take care of all timers
related to the interface.

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>