mlx5: fix error return code in mlx5_alloc_uuars()

Fix to return -ENOMEM from the ioremap error handling
case instead of 0, as done elsewhere in this function.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>