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>

drivers/net/ethernet/sfc/mcdi.h

@@ -190,6 +190,7 @@ void efx_mcdi_sensor_event(struct efx_nic *efx, efx_qword_t *ev);
  * 32-bit-aligned.  Also, on Siena we must copy to the MC shared
  * memory strictly 32 bits at a time, so add any necessary padding.
  */
+#define MCDI_TX_BUF_LEN(_len) DIV_ROUND_UP((_len), 4)
 #define _MCDI_DECLARE_BUF(_name, _len)					\
 	efx_dword_t _name[DIV_ROUND_UP(_len, 4)]
 #define MCDI_DECLARE_BUF(_name, _len)					\

drivers/net/ethernet/sfc/ptp.c

@@ -326,7 +326,7 @@ struct efx_ptp_data {
 	struct work_struct pps_work;
 	struct workqueue_struct *pps_workwq;
 	bool nic_ts_enabled;
-	_MCDI_DECLARE_BUF(txbuf, MC_CMD_PTP_IN_TRANSMIT_LENMAX);
+	efx_dword_t txbuf[MCDI_TX_BUF_LEN(MC_CMD_PTP_IN_TRANSMIT_LENMAX)];
 
 	unsigned int good_syncs;
 	unsigned int fast_syncs;