[IRDA]: Support for Sigmatel STIR421x chip

This patch enables support for the Sigmatel's STIR421x IrDA chip.
Once patched with Sigmatel's firmware, this chip "almost" follows the
USB-IrDA spec. Thus this patch is against irda-usb.[ch].

The code has been tested by Nick Fedchik on an STIR4210 chipset based
dongle.

Signed-off-by: Samuel Ortiz <samuel.ortiz@nokia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>