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 <[email protected]>
Acked-by: Nikolay Aleksandrov <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

Author: idosch