mac80211: correct use_short_preamble handling

ERP IE bit for preamble mode is 0 for short and 1 for long, not the other
way around. This fixes the value reported to the driver via
bss_conf->use_short_preamble field.

Signed-off-by: Vladimir Koutny <vlado@ksp.sk>
Signed-off-by: John W. Linville <linville@tuxdriver.com>