mlxsw: spectrum_switchdev: Convert mlxsw_sp_mc_write_mdb_entry() to return int

The function returns 'false' upon failure and 'true' upon success.
Convert it to the usual scheme of returning integer error codes and
align the callers.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>