net: stmmac: make sure that ptp_rate is not 0 before configuring EST

If the ptp_rate recorded earlier in the driver happens to be 0, this
bogus value will propagate up to EST configuration, where it will
trigger a division by 0.

Prevent this division by 0 by adding the corresponding check and error
code.

Suggested-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: Alexis Lothoré <alexis.lothore@bootlin.com>
Fixes: 8572aec ("net: stmmac: Add basic EST support for XGMAC")
Link: https://patch.msgid.link/20250529-stmmac_tstamp_div-v4-2-d73340a794d5@bootlin.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Tropicao

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

@@ -32,6 +32,11 @@ static int est_configure(struct stmmac_priv *priv, struct stmmac_est *cfg,
 	int i, ret = 0;
 	u32 ctrl;
 
+	if (!ptp_rate) {
+		netdev_warn(priv->dev, "Invalid PTP rate");
+		return -EINVAL;
+	}
+
 	ret |= est_write(est_addr, EST_BTR_LOW, cfg->btr[0], false);
 	ret |= est_write(est_addr, EST_BTR_HIGH, cfg->btr[1], false);
 	ret |= est_write(est_addr, EST_TER, cfg->ter, false);