[netdrvr] sfc: Add phy_flash_cfg module parameter and implementation

The 10Xpress PHY supports flash upgrades through MDIO, but needs to be
put in upgrade mode at power-up.  This adds a module parameter and other
logic to support that.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>