ser_gigaset: return -ENOMEM on error instead of success

If we can't allocate the resources in gigaset_initdriver() then we
should return -ENOMEM instead of zero.

Fixes: 2869b23e ("[PATCH] drivers/isdn/gigaset: new M101 driver (v2)")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>