ACPI: bus: Constify acpi_companion_match() returned value

acpi_companion_match() doesn't alter the contents of the passed
parameter, so we don't expect that returned value can be altered
either. So constify it.

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