[MAC80211]: fix preamble setting

It looks like in commit 28487a90 the condition was unintentionally
negated by moving some code, fix it.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Cc: Daniel Drake <dsd@gentoo.org>
Acked-by: Michael Wu <flamingice@sourmilk.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>