ath9k: fix invalid pointer access in the tx path

commit 80b08a8d8829a58b5db14b1417151094cc28face upstream.

After setup_frame_info has been called, only info->control.rates is still
valid, other control fields have been overwritten by the ath_frame_info
data. Move the access to info->control.vif for checking short preamble
to setup_frame_info before it gets overwritten.

This regression was introduced in commit d47a61aa
"ath9k: Fix multi-VIF BSS handling"

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Reported-by: Thomas Hühn <thomas@net.t-labs.tu-berlin.de>
Acked-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>