isdn: hisax: Handle return value of pnp_irq and pnp_port_start

pnp_irq() and pnp_port_start() can fail here and we must check
its return value.

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