b43: Fix PIO skb clobber

This fixes a clobber of the skb that was introduced by the
tx_control->cb conversion patches.
This bug causes a crash when the skb destructor is invoked. That happens
on skb_orphan or skb_kfree.

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>