USB: cdc-wdm: cannot use dev_printk when device is gone
We cannot dereference a removed USB interface for
dev_printk. Use pr_debug instead where necessary.
Flush errors are expected if device is unplugged and are
therefore best ingored at this point.
Move the kill_urbs() call in wdm_release with dev_dbg()
for the non disconnect, as we know it has already been
called if WDM_DISCONNECTING is set. This does not
actually fix anything, but keeps the code more consistent.