usb: gadget: u_serial: clear suspended flag when disconnecting

The commit aba3a8d0 ("usb: gadget: u_serial: add suspend resume
callbacks") set/cleared the suspended flag in USB bus suspend/resume
only. But, when a USB cable is disconnected in the suspend, since some
controllers will not detect USB bus resume, the suspended flag is not
cleared. After that, user cannot send any data. To fix the issue,
clears the suspended flag in the gserial_disconnect().

Fixes: aba3a8d0 ("usb: gadget: u_serial: add suspend resume callbacks")
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Tested-by: Linh Phung <linh.phung.jy@renesas.com>
Tested-by: Tam Nguyen <tam.nguyen.xa@renesas.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>