bnx2x: Fix warning message during 57712/8727 initialization

Clear warning message "MDC/MDIO access timeout" during first driver load by setting MDIO clock.
This warning has no significant meaning, since it occurs prior to the first PHY initialization.

Signed-off-by: Yaniv Rosner <yanivr@broadcom.com>
Signed-off-by: Eilon Greenstein <eilong@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>