smsc911x: allow mac address to be saved before device reset

Some platforms (for example pcm037) do not have an EEPROM fitted,
instead storing their mac address somewhere else.  The bootloader
fetches this and configures the ethernet adapter before the kernel is
started.

This patch allows a platform to indicate to the driver via the
SMSC911X_SAVE_MAC_ADDRESS flag that the mac address has already been
configured via such a mechanism, and should be saved before resetting
the chip.

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>