ath5k: Include tx ack reporting on hw flags

 * Since we report tx acks to the protocol stack, add the needed
 flag to hw_flags. This way we'll also use the new AP probing mechanism.

Signed-off-by: Nick Kossifidis <mickflemm@gmail.com>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>