usb: roles: fix NULL pointer issue when put module's reference

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

usb: roles: fix NULL pointer issue when put module's reference

In current design, usb role class driver will get usb_role_switch parent's

module reference after the user get usb_role_switch device and put the

reference after the user put the usb_role_switch device. However, the

parent device of usb_role_switch may be removed before the user put the

usb_role_switch. If so, then, NULL pointer issue will be met when the user

put the parent module's reference.

This will save the module pointer in structure of usb_role_switch. Then,

we don't need to find module by iterating long relations.