sfc: add suffix to large constant in ptp

Fixes: 1280c0f8 ("sfc: support second + quarter ns time format for receive datapath")
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Bert Kenward <bkenward@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>