ice: Stop processing VF messages during teardown

When VFs are setup and torn down in quick succession, it is possible
that a VF is torn down by the PF while the VF's virtchnl requests are
still in the PF's mailbox ring. Processing the VF's virtchnl request
when the VF itself doesn't exist results in undefined behavior. Fix
this by adding a check to stop processing virtchnl requests when VF
teardown is in progress.

Fixes: ddf30f7f ("ice: Add handler to configure SR-IOV")
Signed-off-by: Anirudh Venkataramanan <anirudh.venkataramanan@intel.com>
Tested-by: Konrad Jankowski <konrad0.jankowski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>