usb: ehci: mark unlink_empty_async_suspended() as __maybe_unused

Patch 4d053fda "usb: ehci: unlink_empty_async_suspended() only used
with CONFIG_PM" tried to hide the unlink_empty_async_suspended function
inside of an #ifdef to work around an unused function warning.

Unfortunately that had the effect of introducing a new warning:

drivers/usb/host/ehci-q.c:1297:13: warning: 'unlink_empty_async_suspended'
	declared 'static' but never defined [-Wunused-function]

While we could add another #ifdef around the function declaration to avoid
this, a nicer solution is to mark it as __maybe_unused, which will let
gcc silently drop the function definition when it is not needed.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>