[IrDA]: Calling ppp_unregister_channel() from process context

We need to call ppp_unregister_channel() when IrNET disconnects, and this
must be done from a process context.

Bug reported and patch tested by Guennadi Liakhovetski.

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