ar71xx: fix up alignment handling

ar71xx: fix up alignment handling

- on ar724x, rx buffers can be aligned with an offset of 2, which keeps the ip header aligned
- alignment offset is only added if the ar8216 workaround is not active and the phy driver does not advertise its own packet alignment
- ar71xx and ar91xx can not handle rx alignment offsets, however taking a hit on unaligned exceptions seems to have less overhead than re-aligning the data for large packets
- use memmove to re-align small packets, if necessary

tested on ar9132, ar7240 and ar7242 based devices without ar8216 headers

SVN-Revision: 20892