mgmt/mcumgr: Fix serial packet length not including CRC16

The length field of packet has not been including the length of
CRC16 field.

Fixes #39546

Signed-off-by: Dominik Ermel <[email protected]>

Author: de-nordic

subsys/mgmt/mcumgr/serial_util.c

@@ -215,7 +215,8 @@ int mcumgr_serial_tx_frame(const uint8_t *data, bool first, int len,
 	 * byte long is paired with first byte of input buffer to form triplet for Base64 encoding.
 	 */
 	if (first) {
-		u16 = sys_cpu_to_be16(len);
+		/* The size of the CRC16 should be added to packet length */
+		u16 = sys_cpu_to_be16(len + 2);
 		memcpy(raw, &u16, sizeof(u16));
 		raw[2] = data[0];