HID: wiimote: init EXT/MP during device detection

We normally get EXT hotplug events or poll for MP hotplugging so we
don't need to force extension port initialization during device setup.
But for gen20 devices, we disable MP polling because MP is always
present. However, this prevents MP initialization during device setup
and users need to plug another extension to trigger EXT/MP detection.

Therefore, we now trigger EXT/MP detection during device setup
automatically. This also avoids slightly delayed extension detection
and provides sysfs child-devices prior to the "changed"-uevent during
device setup.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>