serial: mcf: use helpers in mcf_tx_chars()

Use uart_circ_empty() instead of open-coding it via xmit->head & tail.
Use preexisting mcf_stop_tx() to avoid stop-tx code duplication.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20220124071430.14907-12-jslaby@suse.cz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>