[IrDA] af_irda: IRDA_ASSERT cleanups

In af_irda.c, the multiple IRDA_ASSERT() are either hiding bugs, useless, or
returning the wrong value.
Let's clean that up.

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