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