um: vector: Fix an error handling path in 'vector_parse()'

If 'find_device()' finds something, we set '*error_out' and we should
return an error. However, 'err' is known to be 0 at this point.

Explicitly return -EINVAL instead.

While at it, remove the initialization of 'err' at the beginning of the
function and also explicitly return an error code if the first check
fails.

Fixes: ad1f62ab2bd4 ("High Performance UML Vector Network Driver")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Richard Weinberger <richard@nod.at>