USB: io_ti, stop dereferencing potential NULL

tty_port_tty_get might return a tty which is NULL. But it is
dereferenced unconditionally in edge_send. Stop dereferencing that by
sending usb_serial_port pointer around.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>