cpmac: convert to napi_struct interface

Convert cpmac to new napi_struct API introduced by
bea3348e [NET]: Make NAPI polling independent
of struct net_device objects.
Only disable rx interrupts if napi actually has been scheduled.

Signed-off-by: Eugene Konev <ejka@imfi.kspu.ru>
Signed-off-by: Jeff Garzik <jeff@garzik.org>