Fix the tx interrupt handler to free completed tx descriptors even
when NAPI is enabled.  Otherwise, the tx queue would fill up resulting
in poor performance and "NETDEV WATCHDOG: <iface>: transmit timed out"
messages.
Signed-off-by: Brent Cook <bcook@bpointsys.com>
Signed-off-by: Dale Farnsworth <dale@farnsworth.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>