netfilter: bridge: refactor clearing BRNF_NF_BRIDGE_PREROUTING

use binary AND on complement of BRNF_NF_BRIDGE_PREROUTING to unset
bit in nf_bridge->mask.

Signed-off-by: Bernhard Thaler <bernhard.thaler@wvnet.at>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>