isdn: hisax: Fix pnp_irq's error checking for setup_sedlbauer_isapnp

The pnp_irq() function returns -1 if an error occurs.
pnp_irq() error checking for zero is not correct.

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>