Input: uinput - uinput_validate_absbits() cleanup

This moves basic checks and setup from uinput_setup_device() into
uinput_validate_absbits() to make it easier to use. This way, we can call
it from other places without copying the boilerplate code.

Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>