iwlwifi: dvm: take mutex when sending SYNC BT config command

There is a flow in which we send the host command in SYNC
mode, but we don't take priv->mutex.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1046495



Cc: <stable@vger.kernel.org>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>