net: stmmac: add aux timestamps fifo clearance wait

Add timeout polling wait for auxiliary timestamps snapshot FIFO clear bit
(ATSFC) to clear. This is to ensure no residue fifo value is being read
erroneously.

Fixes: f4da5652 ("net: stmmac: Add support for external trigger timestamping")
Cc: <stable@vger.kernel.org> # 5.10.x
Signed-off-by: Noor Azura Ahmad Tarmizi <noor.azura.ahmad.tarmizi@intel.com>
Link: https://lore.kernel.org/r/20230111050200.2130-1-noor.azura.ahmad.tarmizi@intel.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/stmicro/stmmac/stmmac_ptp.c

@@ -210,7 +210,10 @@ static int stmmac_enable(struct ptp_clock_info *ptp,
 		}
 		writel(acr_value, ptpaddr + PTP_ACR);
 		mutex_unlock(&priv->aux_ts_lock);
-		ret = 0;
+		/* wait for auxts fifo clear to finish */
+		ret = readl_poll_timeout(ptpaddr + PTP_ACR, acr_value,
+					 !(acr_value & PTP_ACR_ATSFC),
+					 10, 10000);
 		break;
 
 	default: