Fix the following -Werror warning from clang 10.0.0 in bsnmpd:

usr.sbin/bsnmpd/modules/snmp_bridge/bridge_port.c:1235:43: error: overlapping comparisons always evaluate to true [-Werror,-Wtautological-overlap-compare]

begemotBridgeStpPortEnable_enabled ||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~

Work around it by casting the enum values to the type of val->v.integer.

MFC after: 3 days