soc: ixp4xx: npe: Fix an IS_ERR() vs NULL check in probe

The devm_ioremap_resource() function doesn't return NULL, it returns
error pointers.

Fixes: 0b458d7b ("soc: ixp4xx: npe: Pass addresses as resources")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Olof Johansson <olof@lixom.net>