update sock api in lantiq atm driver

The ltq-atm driver provides its own replacement for net/atm/common.c:alloc_tx(),
defined in ltq_atm.c:atm_alloc_tx().
The read of sk_wmem_alloc is done in the wrong way, starting with commit
2b85a34e911bf483c27cfdd124aeb1605145dc80 in linux mainline it has to be
done through a specific wrapper.

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@35991 3c298f89-4303-0410-b956-a3cf2f4a3e73