While we're here improving virtchnl we can include two minor fixes for
the lower level ctrlq flow.

This adds a memory barrier to idpf_post_rx_buffs before we update tail
on the controlq.  We should make sure our writes have had a chance to
finish before we tell HW it can touch them.

This also removes some defensive programming in idpf_ctrlq_recv. The
caller should not be using a num_q_msg value of zero or more than the
ring size and it's their responsibility to call functions sanely.
Tested-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Signed-off-by: Alan Brady <alan.brady@intel.com>
Tested-by: Krishneil Singh <krishneil.k.singh@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>