net: stmmac: ptp: fix potentially overflowing expression

Convert the u32 variable to type u64 in a context where expression of
type u64 is required to avoid potential overflow.

Fixes: e9e37200 ("net: stmmac: ptp: update tas basetime after ptp adjust")
Signed-off-by: Xiaoliang Yang <xiaoliang.yang_1@nxp.com>
Link: https://lore.kernel.org/r/20211223073928.37371-1-xiaoliang.yang_1@nxp.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

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

@@ -102,7 +102,7 @@ static int stmmac_adjust_time(struct ptp_clock_info *ptp, s64 delta)
 		time.tv_nsec = priv->plat->est->btr_reserve[0];
 		time.tv_sec = priv->plat->est->btr_reserve[1];
 		basetime = timespec64_to_ktime(time);
-		cycle_time = priv->plat->est->ctr[1] * NSEC_PER_SEC +
+		cycle_time = (u64)priv->plat->est->ctr[1] * NSEC_PER_SEC +
 			     priv->plat->est->ctr[0];
 		time = stmmac_calc_tas_basetime(basetime,
 						current_time_ns,