ACPI / osl: Refactor acpi_os_get_root_pointer() to drop 'else':s

There are few 'else' keywords which are redundant in
acpi_os_get_root_pointer(). Refactor function to get rid of them.

While here, switch to pr_err() instead of printk(KERN_ERR ...).

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>