usb: serial: prevent suspend/resume from racing against probe/remove

Some usb-serial drivers may access port data in their suspend/
resume functions. Such drivers must always verify the validity
of the data as both suspend and resume can be called both before
usb_serial_device_probe and after usb_serial_device_remove.

But the port data may be invalidated during port_probe and
port_remove. This patch prevents the race against suspend and
resume by disabling suspend while port_probe or port_remove is
running.

Suggested-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>