USB: gadget: Remove suspended sysfs file before freeing cdev

cdev struct is accessed in suspended sysfs show function.  So
remove sysfs file before freeing the cdev in composite_unbind.

Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>