bgmac: leave interrupts disabled as long as there is work to do

Always poll rx and tx during NAPI poll instead of relying on the status
of the first interrupt. This prevents bgmac_poll from leaving unfinished
work around until the next IRQ.
In my tests this makes bridging/routing throughput under heavy load more
stable and ensures that no new IRQs arrive as long as bgmac_poll uses up
the entire budget.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>