net/mlx4_core: Fix misplaced brackets of sizeof

When changing the sizeof style usage in the patch cited below,
one brackets misplacement was introduced. Here we fix it.

Fixes: 31975e27 ("mlx4: sizeof style usage")
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Cc: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>