sfc: make function tables const

The phy, mac, and board information structures should be const.
Since tables contain function pointer this improves security
(at least theoretically).

Compile tested only.

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