sfc: Maintain current frequency adjustment when applying a time offset

There is a single MCDI PTP operation for setting the frequency
adjustment and applying a time offset to the hardware clock.  When
applying a time offset we should not change the frequency adjustment.

These two operations can now be requested separately but this requires
a flash firmware update.  Keep using the single operation, but
remember and repeat the previous frequency adjustment.

Fixes: 7c236c43b838 ('sfc: Add support for IEEE-1588 PTP')
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>

drivers/net/ethernet/sfc/ptp.c

@@ -1426,7 +1426,7 @@ static int efx_phc_adjfreq(struct ptp_clock_info *ptp, s32 delta)
 	if (rc != 0)
 		return rc;
 
-	ptp_data->current_adjfreq = delta;
+	ptp_data->current_adjfreq = adjustment_ns;
 	return 0;
 }
 
@@ -1441,7 +1441,7 @@ static int efx_phc_adjtime(struct ptp_clock_info *ptp, s64 delta)
 
 	MCDI_SET_DWORD(inbuf, PTP_IN_OP, MC_CMD_PTP_OP_ADJUST);
 	MCDI_SET_DWORD(inbuf, PTP_IN_PERIPH_ID, 0);
-	MCDI_SET_QWORD(inbuf, PTP_IN_ADJUST_FREQ, 0);
+	MCDI_SET_QWORD(inbuf, PTP_IN_ADJUST_FREQ, ptp_data->current_adjfreq);
 	MCDI_SET_DWORD(inbuf, PTP_IN_ADJUST_SECONDS, (u32)delta_ts.tv_sec);
 	MCDI_SET_DWORD(inbuf, PTP_IN_ADJUST_NANOSECONDS, (u32)delta_ts.tv_nsec);
 	return efx_mcdi_rpc(efx, MC_CMD_PTP, inbuf, sizeof(inbuf),