USB: OHCI: at91: remove useless extern declaration

Remove usb_disabled() extern declaration as it is already declared
as extern in include/linux/usb.h.

Signed-off-by: Manjunath Goudar <csmanjuvijay@gmail.com>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-usb@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>