usb: renesas_usbhs: modify the irq handler for sharing irq

When IORESOURCE_IRQ_SHAREABLE is set, the irq handler may be called
even if the interupt of the USB module doesn't happen. So, it may
clear the interrupt flags by mistake. This patch fixes it.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>