selftests: bridge_mdb: Use MDB get instead of dump

Test the new MDB get functionality by converting dump and grep to MDB
get.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: David S. Miller <davem@davemloft.net>