usb: r8a66597-udc disable interrupts on shutdown fix

This patch improves the disable_controller() function in the
r8a66597-udc driver to disable all interrupts and also clear
status flags. With this patch in place the driver survives
kexec.

Signed-off-by: Magnus Damm <damm@igel.co.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>