acpi: Replace weird use of PTR_RET.

This functions is really weird.  It sets rc to -ENOMEM, then overrides
it.  It was converted to PTR_RET in a1458187 when it should have
simply been rewritten.

This version makes it more explicit, with a single IS_ERR() test.

Cc: Alexandru Gheorghiu <gheorghiuandru@gmail.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>