sh: mach-ecovec24: deassert usb irq on boot.

This patch prevents the USB1 interrupt from remaining asserted
immediately after re-boot or during a jump in to a secondary kernel.

Signed-off-by: Kuninori Morimoto <morimoto.kuninori@renesas.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>