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