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.

Cc: Oliver Neukum <oliver@neukum.org>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>