ath6kl: Fix connect command to clear previously used IEs

Empty IE buffer means that the new association is not supposed to
include extra IEs. Make sure any previously configured (Re)Association
Request frame IEs get cleared in such a case. This is based on a patch
from Shuibing.

Cc: Dai Shuibing <shuibing@qca.qualcomm.com>
Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>