Input: iforce - only call iforce_process_packet() if initialized

It is excessive to check if device is fully initialized in
iforce_process_packet(), as for USB-conected devices we do not start
collecting reports until the device is fully initialized.

Let's change serio transport code to not call iforce_process_packet()
until device initialization is done.

Tested-by: Tim Schumacher <timschumi@gmx.de>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>