[IrDA]: tsap init routine factorisation.

This patch extracts common code from irttp_open_tsap() and irttp_dup()
into a new function to 1) avoid code duplication, 2) help avoid
forgetting object initialization in the tsap duplication path in the
future.

Signed-off-by: G. Liakhovetski <gl@dsa-ac.de>
Signed-off-by: Samuel Ortiz <samuel@sortiz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>