sfc: fix kdoc warning

kernel-doc script as used by W=1, is confused by the macro
usage inside the header describing the efx_ptp_data struct.

drivers/net/ethernet/sfc/ptp.c:345: warning: Function parameter or member 'MC_CMD_PTP_IN_TRANSMIT_LENMAX' not described in 'efx_ptp_data'

After some discussion on the list, break this patch out to
a separate one, and fix the issue through a creative
macro declaration.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Acked-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>