tty: serial: extract serial_omap_put_char() from transmit_chars()

This non-trivial code is doubled in transmit_chars(), so it deserves its
own function. This will make next patches easier.

Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20220920052049.20507-8-jslaby@suse.cz


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