cpmac: wait longer after MDIO reset

This patch slows down the MDIO_ALIVE busy waiting to let
switches and PHY come up after reset. Previous loop was
too quick for IC+175C and ADM6996C/L switches to come up.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>