platform/chrome: cros_ec_rpmsg: Document missing struct parameters

Kerneldoc expects all kernel structure member to be documented.

Fixes the following W=1 level warnings:

  cros_ec_rpmsg.c:49: warning: Function parameter or member 'ept' not described in 'cros_ec_rpmsg'
  cros_ec_rpmsg.c:49: warning: Function parameter or member 'has_pending_host_event' not described in 'cros_ec_rpmsg'
  cros_ec_rpmsg.c:49: warning: Function parameter or member 'probe_done' not described in 'cros_ec_rpmsg'

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Reviewed-by: Gwendal Grignou <gwendal@chromium.org>