brcm63xx: fix ethernet breakage in 3.18

Linux commit d75b1ade567ffab ("net: less interrupt masking in NAPI")
chose the samantics of the napi poll return value, which caused napi to
not resume interrups/polling anymore.

Fix the return value to restore expected behaviour.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@44576 3c298f89-4303-0410-b956-a3cf2f4a3e73