net/mlx4_core: Add missing iounmap() in error path

This fixes the following coccicheck warning:

drivers/net/ethernet/mellanox/mlx4/crdump.c:200:2-8: ERROR: missing iounmap;
ioremap on line 190 and execution via conditional on line 198

Fixes: 7ef19d3b ("devlink: report error once U32_MAX snapshot ids have been used")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zou Wei <zou_wei@huawei.com>
Reviewed-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>