sfc: Always start Falcon using the XMAC

The strap bits are only important on Falcon A and all production
boards using it have fixed-speed 10G PHYs.

Replace dummy MAC operations with default MAC operations.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>