usb: ehci: unlink_empty_async_suspended() only used with CONFIG_PM

Compiling with !CONFIG_PM generates an unused function warning on
unlink_empty_async_suspended().

Enclose the function in a #ifdef CONFIG_PM

Signed-off-by: Tony Prisk <linux@prisktech.co.nz>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>