IB/mthca: Fix memory leak of multicast group structures

Convert "/ (1 << lg)" to ">> lg" for a slight code size reduction.

add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-24 (-24)
function                                     old     new   delta
mthca_map_cmd                                613     589     -24

Signed-off-by: Ishai Rabinovitz <ishai@mellanox.co.il>
Signed-off-by: Michael S. Tsirkin <mst@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>