mfd: cros_ec_dev: Fix cros_feature_to_{name,cells} struct descriptions

Kerneldoc expects kernel structures to be prefixed with 'struct'.

Fixes the following W=1 level warnings:

 drivers/mfd/cros_ec_dev.c:32: warning: cannot understand function prototype: 'struct cros_feature_to_name '
 drivers/mfd/cros_ec_dev.c:44: warning: cannot understand function prototype: 'struct cros_feature_to_cells '

Cc: Benson Leung <bleung@chromium.org>
Cc: Guenter Roeck <groeck@chromium.org>
Cc: Bill Richardson <wfrichar@chromium.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>