avf: fix race between avf process node and avf_delete_if(...)

avf: fix race between avf process node and avf_delete_if(...)

It may happen that process node is suspended while it waits for response
from adminq and during that time CLI or API process can call
avf_delete_if. When avf process node resumes, it may happen that device
is not there anymeore.

This patch delegates interface deletion to process node, so CLI/API
process just sends signal instead of deleting device instance itself.

Type: fix

Change-Id: I7f12e12df3071650f6e60ad7eb5af23b7acfe335
Signed-off-by: Damjan Marion <damarion@cisco.com>