platform-drivers-x86: ideapad-laptop: add missing ideapad_input_exit in ideapad_acpi_add error path

In the case of ideapad_backlight_init() failure,
we need to free the resources allocated by ideapad_input_init().

Aslo drop __devexit annotation for ideapad_input_exit() because
we also call it in ideapad_acpi_add() error path.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>