tty: serial: owl-uart, send x_char even if stopped

Flow control characters should be sent even if the TX is stopped. So fix
owl-uart to behave the same as other drivers.

This unification also allows the use of the TX helper in the future.

Cc: "Andreas Färber" <afaerber@suse.de>
Cc: Manivannan Sadhasivam <mani@kernel.org>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20220411104506.8990-3-jslaby@suse.cz


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