p54: fix skb->cb tx info conversion

When I moved the TX info into skb->cb apparently I forgot to change a
few places to put the p54-internal data into info->driver_data rather
than skb->cb. This should fix it.

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