net: dsa: yt921x: Fix MIB overflow wraparound routine

Reported by the following Smatch static checker warning:

drivers/net/dsa/yt921x.c:702 yt921x_read_mib()
warn: was expecting a 64 bit value instead of '(~0)'

Fixes: 186623f4aa72 ("net: dsa: yt921x: Add support for Motorcomm YT921x")
Reported-by: Dan Carpenter <[e-mail]>
Closes: https://lore.kernel.org/netdev/[e-mail]/
Suggested-by: David Laight <[e-mail]>
Signed-off-by: David Yang <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/net/dsa/yt921x.c

···
 		const struct yt921x_mib_desc *desc = &yt921x_mib_descs[i];
 		u32 reg = YT921X_MIBn_DATA0(port) + desc->offset;
 		u64 *valp = &((u64 *)mib)[i];
-		u64 val = *valp;
 		u32 val0;
-		u32 val1;
+		u64 val;
 
 		res = yt921x_reg_read(priv, reg, &val0);
 		if (res)
 			break;
 
 		if (desc->size <= 1) {
-			if (val < (u32)val)
-				/* overflow */
-				val += (u64)U32_MAX + 1;
-			val &= ~U32_MAX;
-			val |= val0;
+			u64 old_val = *valp;
+
+			val = (old_val & ~(u64)U32_MAX) | val0;
+			if (val < old_val)
+				val += 1ull << 32;
 		} else {
+			u32 val1;
+
 			res = yt921x_reg_read(priv, reg + 4, &val1);
 			if (res)
 				break;