ath5k: use short preamble when possible

ath5k previously ignored TX_RC_SHORT_PREAMBLE and did not use
config->use_short_preamble, so the long preamble was always
used for transmitted packets.

Changes-licensed-under: 3-Clause-BSD

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