b43legacy: fix build errors when DMA or PIO are not selected

Currently, b43legacy is broken due to

commit fbad4598ca826b994d0fd4ce3deebc9cd1960b31
Author: Johannes Berg <johannes@sipsolutions.net>
Date:   Thu May 15 12:55:29 2008 +0200

    mac80211: move TX info into skb->cb

when compiled with only PIO or only DMA because I forgot to update two
stubs. This patch fixes it.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>