usb: gadget: u_serial: Add null pointer check in gserial_resume

In the Linux kernel, the following vulnerability has been resolved:

usb: gadget: u_serial: Add null pointer check in gserial_resume

Consider a case where gserial_disconnect has already cleared gser->ioport. And if a wakeup interrupt triggers afterwards, gserial_resume gets called, which will lead to accessing of gser->ioport and thus causing null pointer dereference.Add a null pointer check to prevent this.

Added a static spinlock to prevent gser->ioport from becoming null after the newly added check.