ACPI / osl: remove an unneeded NULL check

"str" is never NULL here so I have removed the check.  There are static
checkers which complain about superfluous NULL checks because it may
indicate confusion or a bug.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>