ACPI / bus: Do not traverse through non-existed device table

When __acpi_match_device() is called it would be possible to have
ACPI ID table a NULL pointer. To avoid potential dereference,
check for this before traverse.

While here, remove redundant 'else'.

Note, this patch implies a bit of refactoring acpi_of_match_device()
to return pointer to OF ID when matched followed by refactoring
__acpi_match_device() to return either ACPI or OF ID when matches.

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