USB: serial: ti_usb_3410_5052: remove useless NULL-testing

It is useless to check the return of usb_get_serial_port_data in the tty
and tty-port callbacks.

No need to check interface private data in close() either.

Signed-off-by: Mathieu OTHACEHE <m.othacehe@gmail.com>
[ johan: amend commit message ]
Signed-off-by: Johan Hovold <johan@kernel.org>