ath5k: use correct packet type when transmitting

The hardware needs to know what type of frames are being
sent in order to fill in various fields, for example the
timestamp in probe responses (before this patch, it was
always 0).  Set it correctly when initializing the TX
descriptor.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Cc: stable@kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>