ralink: fix ethernet feature TSO not work

* fix TSO features verify on mt7621 firewrt board
* improve tx clean up. no need to access uncached
  memory. also use TX_DTX register instead of
  read tx ring DONE bit
* mt7621 need napi weight 64 to get more performance
* remove netif_receive_skb, after kernel version
  3.7 tcp4_gro_receive can handle tcp checksum.
  on rt2880 use iperf tcp LAN to WAN throughput test.
  with gro 135 Mbits/sec. without gro 80.4Mbits/sec.

Signed-off-by: michael lee <igvtee@gmail.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@44118 3c298f89-4303-0410-b956-a3cf2f4a3e73