lantiq: atm: Use a tasklet to handle incoming packets. Fix #12917.

Incoming packets are now processes in a tasklet instead of in the
irq handler; this should improve latency.

This patch is based on a previous version of ltq-atm driver, which
did implement a tasklet.

It has been tested on a arv4518pw with a
Lantiq Danube for about a month and it seems to work well.

Signed-off-by: Luca Dariz <luca.dariz@gmail.com>
Tested-by: Luca Dariz <luca.dariz@gmail.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@36902 3c298f89-4303-0410-b956-a3cf2f4a3e73