bcm63xx_enet: timeout off by one in do_mdio_op()

`while (limit-- >= 0)' reaches -2 after the loop upon timeout.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Acked-by: Maxime Bizon <mbizon@freebox.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>