mptcp: fill in missing MODULE_DESCRIPTION()

W=1 builds warn on missing MODULE_DESCRIPTION, add them here in MPTCP.

Only two were missing: two modules with different KUnit tests for MPTCP.

Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts <matttbe@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>