smsc911x: add external phy detection overrides

On LAN9115/LAN9117/LAN9215/LAN9217, external phys are supported.  These
are usually indicated by a hardware strap which sets an "external PHY
detected" bit in the HW_CFG register.

In some cases it is desirable to override this hardware strap and force
use of either the internal phy or an external PHY.  This patch adds
SMSC911X_FORCE_INTERNAL_PHY and SMSC911X_FORCE_EXTERNAL_PHY flags so a
platform can indicate this preference via its platform_data.

Signed-off-by: Steve Glendinning <steve.glendinning@smsc.com>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Tested-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>