ramips: fix ethernet driver tx completion polling

- budget is decremented with completed frames, so don't check if done is
smaller
- ACK the interrupt before processing further frames to fix a small race
condition.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@44234 3c298f89-4303-0410-b956-a3cf2f4a3e73