atl1: zero out CMB and SBM in atl1_free_ring_resources

They are allocated in atl1_setup_ring_resources, zero out the pointers
in atl1_free_ring_resources (like the other resources).

Signed-off-by: Luca Tettamanti <kronos.it@gmail.com>
Acked-by: Chris Snook <chris.snook@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>