USB: count reaches -1, tested 0

With a postfix decrement count will reach -1 rather than 0,
so the warning will not be issued.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>