platform/x86: mlx-platform: Fix an ERR_PTR vs NULL issue

devm_ioport_map() returns NULL on error but we accidentally check for
error pointers instead.

Fixes: c6acad68

 ("platform/mellanox: mlxreg-hotplug: Modify to use a regmap interface")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Vadim Pasternak <vadimp@melanox.com>
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>