I2C transport: Fix check for Byte Count field and packet length

The complete packet includes destination address,
command code, byte count and source address.
The check previously accounted only for the source adddress,
while the checked packet includes the whole header at that point.

Signed-off-by: Marvin Gudel <marvin.gudel@9elements.com>

Author: embediver

mctp-estack/src/i2c.rs

@@ -106,8 +106,10 @@ impl MctpI2cEncap {
 
         let header =
             MctpI2cHeader::decode(packet.get(..4).ok_or(Error::InvalidInput)?)?;
-        // +1 for i2c source address field
-        if header.byte_count != packet.len() + 1 {
+        // total packet len == byte_count + 3 (destination, command code, byte count)
+        // pec is not included
+        if header.byte_count + 3 != packet.len() {
+            trace!("Packet byte count mismatch");
             return Err(Error::InvalidInput);
         }