soc: fsl: guts: fix return value check in fsl_guts_init()

In case of error, of_iomap() returns NULL pointer not ERR_PTR().
The IS_ERR() test in the return value check should be replaced
with NULL test and return -ENOMEM as error value.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Michael Walle <michael@walle.cc>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>