mfd: cros_ec: Use struct cros_ec_command to communicate with the EC

This is some internal structure reorganization / renaming to prepare
for future patches that will add a userspace API to cros_ec.  There
should be no visible changes.

Signed-off-by: Bill Richardson <wfrichar@chromium.org>
Signed-off-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>