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

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>
SVN-Revision: 36902