mlxsw: spectrum_switchdev: Handle error in mlxsw_sp_bridge_mdb_mc_enable_sync()

The above mentioned function calls two functions which return values, but
it ignores them.

Check if the return value is an error, handle it in such case and return
an error back to the caller.

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>