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 2be818a116b2 ("Revert "USB: quatech2: only write to the tty if the port is open."")

Signed-off-by YueHaibing <[email protected]>