net: renesas: ravb: unintialized return value

We want to set the other "err" variable here so that we can return it
later.  My version of GCC misses this issue but I caught it with a
static checker.

Fixes: 9f70eb33 ("net: ethernet: renesas: ravb: fix fixed-link phydev leaks")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Johan Hovold <johan@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>