ar71xx: improve unaligned access patches, add a net_hdr_word() macro which assumes 2-byte aligned access and generates better code than {get,put}_unaligned_cpu32

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@35129 3c298f89-4303-0410-b956-a3cf2f4a3e73