sh_mobile_meram: Reset ICBs at unregistration time

When ICBs are unregistered and later reused they need to be reset to
avoid data corruption. Set the WBF, WF and RF bits to make sure ICBs get
reset properly.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>