Use sizeof(tx_buf_len) instead of length.
Use sizeof(tx_buf_len) instead of length.

* Length should always be 4 but it's safer this way.

openmv/openmv,openmv/openmv,iabdalkader/openmv,iabdalkader/openmv,openmv/openmv,openmv/openmv,kwagyeman/openmv,kwagyeman/openmv,kwagyeman/openmv,iabdalkader/openmv,iabdalkader/openmv,kwagyeman/openmv