Input: iforce - move command completion handling to serio code

Continue teasing apart protocol-specific bits from core into transport
modules. This time move RS232-specific command completion handling
from core to iforce-serio module.

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