tty: cpm_uart, use port->flags instead of low_latency

This is the only in-kernel user of tty_port::low_latency. Switch this
last one to test uport->flags directly as tty_port::low_latency is going
away in the next patch.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210105120239.28031-10-jslaby@suse.cz


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