p54: upgrade memrecord to p54_tx_info

mac80211 reserves 24 bytes in skb->cb for the driver.
So far, we only used them to keep track of used and free device memory.
But p54spi will need a slice of it, as well as the stuck frame detection.

Signed-off-by: Christian Lamparter <chunkeey@web.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>