[IRDA]: stir4200, switching to the kthread API

stir4200 uses a kernel thread for its TX/RX operations, and it is now
converted to the kernel kthread API.
Tested on an STIR4200 based dongle.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Samuel Ortiz <samuel@sortiz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>