PM / devfreq: exynos-nocp: Remove incorrect IS_ERR() check

Smatch complains because platform_get_resource() returns NULL on error
and not an error pointer so the check is wrong.  Julia Lawall pointed
out that normally we don't check these, because devm_ioremap_resource()
has a check for NULL.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>