eeepc-laptop: make input device a child of the platform device

Sysfs showed the ehotk input device as a "virtual" device - lies!
The input device is provided by a physical device, the eeepc platform.

This requires that we move the creation of the input device to come
after platform device is created.  Input initialization is moved from
ehotk_check() [sic] to a new function called eeepc_input_init().  This
brings the input device into line with the other eeepc-laptop devices.

Also, refuse to load if we fail to register the input device.

Signed-off-by: Alan Jenkins <alan-jenkins@tuffmail.co.uk>
Signed-off-by: Corentin Chary <corentincj@iksaif.net>
Signed-off-by: Len Brown <len.brown@intel.com>