wimax: fix oops if netlink fails to add attribute

When sending a message to user space using wimax_msg(), if nla_put()
fails, correctly interpret the return code from wimax_msg_alloc() as
an err ptr and return the error code instead of crashing (as it is
assuming than non-NULL means the pointer is ok).

Signed-off-by: Inaky Perez-Gonzalez <inaky@linux.intel.com>