usb_kbd: use async polling instead of regular polling
This fixes the overpolling of USB keyboard which
causes slowdown of entier barebox by using periodic
polling occuting once per some milliseconds.
Signed-off-by: Aleksey Kuleshov <rndfax@yandex.ru>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>