Blackfin EMAC driver: fix bug - NAT doesn't work with bfin_mac driver

https://blackfin.uclinux.org/gf/project/uclinux-dist/forum/?action=ForumBrowse&forum_id=39&thread_id=23114&_forum_action=ForumMessageBrowse
Today I was dealing with the same problem, on my custom bf537 board, and bfin_mac driver.
I found that the problem is in setting ip_summed flag of skbuff structure,

Signed-off-by: Vitja Makarov <vitja.makarov@gmail.com>
Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Bryan Wu <bryan.wu@analog.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>