sfc: avoid max() in array size

It confuses sparse, which thinks the size isn't constant.  Let's achieve
 the same thing with a BUILD_BUG_ON, since we know which one should be
 bigger and don't expect them ever to change.

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>