[IRDA]: irda-usb.c: STIR421x cleanups

This cleans the STIR421x part of the irda-usb code. We also no longer
try to load all existing firmwares but only the matching one
(according to the USB id we get from the dongle).

Signed-off-by: Nick Fedchik <nfedchik@atlantic-link.com.ua>
Signed-off-by: Samuel Ortiz <samuel@sortiz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>