ethernet :mellanox :mlx4: Replace pci_pool_alloc by pci_pool_zalloc

In mlx4_alloc_cmd_mailbox(), pci_pool_alloc() followed by memset will be
replaced by pci_pool_zalloc()

Signed-off-by: Souptick joarder <jrdr.linux@gmail.com>
Reviewed-by: Yuval Shaia <yuval.shaia@oracle.com>
Reviewed-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>