USB: EHCI: fix compiler warning introduced by commit 2a40f324

Fix the following compiler warning (found by the kbuild test robot):

drivers/usb/host/ehci-hcd.c:312:13: warning: 'unlink_empty_async_suspended' declared 'static' but never defined

Commit 2a40f324 ("USB: EHCI: fix regression during bus resume")
protected the function definition with a "#ifdef CONFIG_PM" block, so
now the declaration needs to be similarly protected.  This patch moves
it to a better location.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>