Input: elan_i2c - enable asynchronous probing

It takes a bit of time to go through controller power up sequence and
initialization. To not stall the overall boot progress let's probe the
controller asynchronously, given that userspace is usually prepared for
hot-plugging of input devices and thus does not rely on particular
ordering.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>