ath5k: set short preamble flag for rx

set the short preamble flag in ieee80211_rx_status for frames received with a
short preamble.

drivers/net/wireless/ath5k/base.c:      Changes-licensed-under: 3-Clause-BSD

Signed-off-by: Bruno Randolf <br1@einfach.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>