mac80211: use short_preamble mode from capability if ERP IE not present

When associating to a b-only AP where there is no ERP IE, short preamble
mode is left at previous state (probably also protection mode). In this
case, disable protection and use short preamble mode as specified in
capability field. The same is done if capability field is changed on-the-fly.

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