ath9k: split out access to tx status information

This patch passes in a pointer to the ath_tx_status data structure for
functions that need it, instead of letting them grab it directly from
the ath_desc struct. This is useful for making it possible to allocate
the intermediate tx status data separately.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>