enic: re-establish V2 VF admin channel and PF registration after reset

The reset paths (enic_reset/enic_tx_hang_reset) tore down and re-opened
the V2 admin/MBOX channel only for the PF: the close/reopen was gated on
enic_sriov_enabled() && vf_type == ENIC_VF_TYPE_V2, which is never true on
a VF (vf_type is set only on the PF; VFs are identified by
enic_is_sriov_vf_v2()).  A VF-initiated reset therefore left the VF admin
QP wiped by the reset but never re-opened, and the VF never re-registered
with the PF, so VF<->PF MBOX traffic (currently link state)
stopped working until the VF was re-probed.

Factor the decision into enic_has_admin_chan() (true for a V2 PF while
SR-IOV is enabled and for every V2 VF) and the reopen sequence into
enic_admin_chan_reopen().  For a VF the helper additionally re-runs the
probe-time handshake (enic_mbox_vf_capability_check() +
enic_mbox_vf_register()) so the PF learns about the VF again; for a PF it
re-pushes the current link state as before.

Before reopening, invalidate the VF's local registration flag.  The reset
only wipes the VF's admin QP, not the PF's software vf_state (that changes
only via the register/unregister MBOX handlers), so the PF may still hold
a stale "registered" until the VF re-registers.  Locally, a failed reopen
or re-handshake must not leave a stale registered state that a later
teardown would try to unregister over a dead channel.

Signed-off-by: Satish Kharat <satishkh@cisco.com>
Link: https://patch.msgid.link/20260812-enic-sriov-v2-admin-channel-v2-v13-11-b3809e448aba@cisco.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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