Fix DPC accounting

Fix DPC accounting

The DPC count is not zeroed when the event count is zeroed leading to some
odd looking stats. Also the DPC count should really be incremented by the
DPC itself to ensure serialization.

While in the neighbourhood, the ring notify functions in transmitter and
receiver look a little superfluous; they can easily be folded directly
into the DPC if the receiver code triggers the transmmitter DPC in the case
of a combined event channel.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>