net/enic: extend VNIC dev API for VF representors

net/enic: extend VNIC dev API for VF representors

VF representors need to proxy devcmd through the PF vnic_dev
instance. Extend vnic_dev to accommodate them as follows.

1. Add vnic_vf_rep_register()
A VF representor creates its own vnic_dev instance via this function
and saves VF ID. When performing devcmd, vnic_dev uses the saved VF ID
to proxy devcmd through the PF vnic_dev instance.

2. Add vnic_register_lock()
As PF and VF representors appear as independent ports to the
application, its threads may invoke APIs on them simultaneously,
leading to race conditions on the PF vnic_dev. For example, thread A
can query stats on PF port, while thread B queries stats on a VF
representor.

The PF port invokes this function to provide a lock to vnic_dev. This
lock is used to serialize devcmd calls from PF and VF representors.

3. Add utility functions to assist VF representor settings
vnic_dev_mtu() and vnic_dev_uif() retrieve vnic MTU and UIF number
(uplink index), respectively.

Signed-off-by: Hyong Youb Kim <hyonkim@cisco.com>
Reviewed-by: John Daley <johndale@cisco.com>