net/mlx4: Mark expected switch fall-through

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

This patch fixes the following warning:

drivers/net/ethernet/mellanox/mlx4/eq.c: In function ‘mlx4_eq_int’:
drivers/net/ethernet/mellanox/mlx4/mlx4.h:219:5: warning: this statement may fall through [-Wimplicit-fallthrough=]
  if (mlx4_debug_level)      \
     ^
drivers/net/ethernet/mellanox/mlx4/eq.c:558:4: note: in expansion of macro ‘mlx4_dbg’
    mlx4_dbg(dev, "%s: MLX4_EVENT_TYPE_SRQ_LIMIT. srq_no=0x%x, eq 0x%x\n",
    ^~~~~~~~
drivers/net/ethernet/mellanox/mlx4/eq.c:561:3: note: here
   case MLX4_EVENT_TYPE_SRQ_CATAS_ERROR:
   ^~~~

Warning level 3 was used: -Wimplicit-fallthrough=3

This patch is part of the ongoing efforts to enabling
-Wimplicit-fallthrough.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>