platform/chrome: cros_usbpd_notify: Fix cros-usbpd-notify notifier

cros-usbpd-notify notifier was returning NOTIFY_BAD when no host event
was available in the MKBP message.
But MKBP messages are used to transmit other information, so return
NOTIFY_DONE instead, to allow other notifier to be called.

Fixes: ec2daf6e ("platform: chrome: Add cros-usbpd-notify driver")
Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Signed-off-by: Benson Leung <bleung@chromium.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>