sfc: siena_check_caps() can be static

Reported-by: Jakub Kicinski <kuba@kernel.org>
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>