USB: serial: quatech2: remove set but not used variable 'port_priv'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/usb/serial/quatech2.c: In function 'qt2_process_read_urb':
drivers/usb/serial/quatech2.c:503:27: warning:
 variable 'port_priv' set but not used [-Wunused-but-set-variable]

It not used any more after commit 2be818a1 ('Revert "USB: quatech2:
only write to the tty if the port is open."')

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Johan Hovold <johan@kernel.org>