net: usb: set error code when usb_alloc_urb fails

In function lan78xx_probe(), variable ret takes the errno code on
failures. However, when the call to usb_alloc_urb() fails, its value
will keeps 0. 0 indicates success in the context, which is inconsistent
with the execution result. This patch fixes the bug, assigning
"-ENOMEM" to ret when usb_alloc_urb() returns a NULL pointer.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=188771

Signed-off-by: Pan Bian <bianpan2016@163.com>
Acked-by: Woojung Huh <woojung.huh@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>