When the driver handles multiple packets per NAPI poll, it is
better to reload the receive ring, then tell the hardware. Otherwise,
under packet storm with flow control, the driver/hardware will degrade
down to one packet getting through per pause-exchange.

Likewise on transmit, don't wakeup until a little more than minimum
ring space is available.
Signed-off-by: Stephen Hemminger <shemminger@osdl.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>