ACPI: scan: Constify acpi_dma_supported() helper function

Constify arguments to acpi_dma_supported(). The function doesn't need
to change the content of the passed argument and when it's const it
allows to supply the result of other functions that may return a pointer
to a constant object.

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