ixgbe: fix potential u32 overflow on shift

The u32 variable rem is being shifted using u32 arithmetic however
it is being passed to div_u64 that expects the expression to be a u64.
The 32 bit shift may potentially overflow, so cast rem to a u64 before
shifting to avoid this.  Also remove comment about overflow.

Addresses-Coverity: ("Unintentional integer overflow")
Fixes: cd458320 ("ixgbe: implement support for SDP/PPS output on X550 hardware")
Fixes: 68d9676f ("ixgbe: fix PTP SDP pin setup on X540 hardware")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/ixgbe/ixgbe_ptp.c

@@ -205,11 +205,8 @@ static void ixgbe_ptp_setup_sdp_X540(struct ixgbe_adapter *adapter)
 	 */
 	rem = (NS_PER_SEC - rem);
 
-	/* Adjust the clock edge to align with the next full second. This
-	 * assumes that the cycle counter shift is small enough to avoid
-	 * overflowing when shifting the remainder.
-	 */
-	clock_edge += div_u64((rem << cc->shift), cc->mult);
+	/* Adjust the clock edge to align with the next full second. */
+	clock_edge += div_u64(((u64)rem << cc->shift), cc->mult);
 	trgttiml = (u32)clock_edge;
 	trgttimh = (u32)(clock_edge >> 32);
 
@@ -291,11 +288,8 @@ static void ixgbe_ptp_setup_sdp_X550(struct ixgbe_adapter *adapter)
 	 */
 	rem = (NS_PER_SEC - rem);
 
-	/* Adjust the clock edge to align with the next full second. This
-	 * assumes that the cycle counter shift is small enough to avoid
-	 * overflowing when shifting the remainder.
-	 */
-	clock_edge += div_u64((rem << cc->shift), cc->mult);
+	/* Adjust the clock edge to align with the next full second. */
+	clock_edge += div_u64(((u64)rem << cc->shift), cc->mult);
 
 	/* X550 hardware stores the time in 32bits of 'billions of cycles' and
 	 * 32bits of 'cycles'. There's no guarantee that cycles represents