net: cpsw: Add missing return value

ret is set 0 at this point, so jumping to that error label would result
in a return value of 0. Set ret to -ENOMEM to return a proper error
value.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Reviewed-by: Wolfram Sang <wsa@the-dreams.de>
Signed-off-by: David S. Miller <davem@davemloft.net>