USB: serial: quatech2: remove unused qt2_setdevice function

clang with W=1 reports
drivers/usb/serial/quatech2.c:179:19: error: unused function
  'qt2_setdevice' [-Werror,-Wunused-function]
static inline int qt2_setdevice(struct usb_device *dev, u8 *data)
                  ^
This function is not used, so remove it.

Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Johan Hovold <johan@kernel.org>