ISDN: Add check for return value of pnp_activate_dev()

pnp_activate_dev() return value needs to be checked to make sure that
following calls calls to the PNP functions do work correctly.
Fix for report #44491 on bugzilla.kernel.org.

Signed-off-by: Karsten Keil <kkeil@linux-pingi.de>
Signed-off-by: David S. Miller <davem@davemloft.net>