The wrong pointer is passed to ieee80211_get_morefrag. Fix this.

While we're at it, reorder things so they look better and the rts duration
calculation is done with the right length.

Thanks to Christoph Hellwig for finding the ieee80211_get_morefrag issue.
Signed-off-by: Michael Wu <flamingice@sourmilk.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>