mlx4: fix warning in drivers/net/mlx4/mcg.c

fix warning:

  drivers/net/mlx4/mcg.c: In function ‘mlx4_multicast_attach’:
  drivers/net/mlx4/mcg.c:217: warning: integer overflow in expression

there was no real danger of overflow here though.

md5:
   db8eb55620f886c03854a2abb2ce6c3f  mcg.o.before.asm
   db8eb55620f886c03854a2abb2ce6c3f  mcg.o.after.asm

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: David S. Miller <davem@davemloft.net>