Now, the tty is not needed at all places in the ISR. So we can just
request in on demand when really needed.

This cleans TX and RX paths a bit as the indentation level can be
dropped by two now when we also invert the char_count if condition.
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>