um: fix error return code in slip_open()

Fix to return a negative error code from the error handling case instead
of 0, as done elsewhere in this function.

Fixes: a3c77c67 ("[PATCH] uml: slirp and slip driver cleanups and fixes")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Acked-By:  <anton.ivanov@cambridgegreys.com>
Signed-off-by: Richard Weinberger <richard@nod.at>