The TC flag is used in function serial_is_tx_ongoing to check if there is
an ongoing serial transmission. So this Flag must not be cleared at the
end of the transmission, otherwise, serial_is_tx_ongoing will notify that
TX is ongoing.
The impact is that it may prevent deep sleep to be entered.
Also there is no need to clear this flag at the end of the transaction
because it will be cleared automatically by HW when a new transmission
starts.