ramips: raeth: add tx_info structure

ramips: raeth: add tx_info structure

This helps to reduce code size on TX path:

add/remove: 0/0 grow/shrink: 1/4 up/down: 56/-92 (-36)
function                                     old     new   delta
ramips_eth_open                             1048    1104     +56
ramips_eth_tx_housekeeping                   156     148      -8
ramips_ring_free                             224     212     -12
ramips_eth_stop                              420     400     -20
ramips_eth_hard_start_xmit                   480     428     -52

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@30731 3c298f89-4303-0410-b956-a3cf2f4a3e73