Gt96k patch correcting the reporting of processed transmit descriptors back to IOS.

Fixes missing interrupt when gt96k doesn't own the first trasmit descriptor.
Fixes incorrect clearing of the Own bit. (last must wait for transmission, the rest are cleared when processed)
Fixes offset when writing back the cmd word of the non-last descriptors. (missing from 0.2.6-RC3)

Affects c2691, c3725 and c3745.
Fixes issue #29 - packet loss with multicast traffic
Forum-link: http://forum.gns3.net/post26454.html#p26454