platform/x86: intel-vbtn: Switch to use devm_input_allocate_device

Use devm_input_allocate_device to simplify the error handling code.
This conversion also makes input_register_device() to be called after
acpi_remove_notify_handler. This avoid a small window that it's possible
to call notify_handler after unregister input device.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>