USB: serial: option: add ZLP support for 0x1bc7/0x9010

Telit FN980 flashing device 0x1bc7/0x9010 requires zero packet
to be sent if out data size is is equal to the endpoint max size.

Signed-off-by: Daniele Palmas <dnlplm@gmail.com>
[ johan: switch operands in conditional ]
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/usb-wwan.h

@@ -38,6 +38,7 @@ struct usb_wwan_intf_private {
 	spinlock_t susp_lock;
 	unsigned int suspended:1;
 	unsigned int use_send_setup:1;
+	unsigned int use_zlp:1;
 	int in_flight;
 	unsigned int open_ports;
 	void *private;