USB: ACPI: Replace usb_acpi_find_port() with acpi_find_child_by_adr()

Instead of walking the list of children of an ACPI device directly
in order to find the child matching a given bus address, use
acpi_find_child_by_adr() for this purpose.

Also notice that if acpi_find_child_by_adr() doesn't find a matching
child, acpi_find_child_device() will not find it too, so directly
replace usb_acpi_find_port() in usb_acpi_get_companion_for_port() with
acpi_find_child_by_adr() and drop it entirely.

Apart from simplifying the code, this will help to eliminate the
children list head from struct acpi_device as it is redundant and it
is used in questionable ways in some places (in particular, locking is
needed for walking the list pointed to it safely, but it is often
missing).

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