USB: serial: ti_usb_3410_5052: remove unused variables

Remove variables affected but never read.

Also drop the now unused TI_SET_SERIAL_FLAGS define.

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