[IrDA] stir4200: removing undocumented bits handling

FIFOCTL_RXERR and FIFOCTL_TXERR are undocumented bits, according to the
Sigmatel datasheet. We should thus not take any assumption on their values
and semantics.

Problem spotted by andrzej zaborowski <balrogg@gmail.com>
Signed-off-by: Samuel Ortiz <samuel@sortiz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>