mfd: cros_ec: Avoid unneeded internal declaration warning

Clang warns:

drivers/mfd/cros_ec_dev.c:509:40: warning: variable 'cros_ec_id' is not
needed and will not be emitted [-Wunneeded-internal-declaration]
static const struct platform_device_id cros_ec_id[] = {
                                       ^
1 warning generated.

Avoid this warning by adding it to the cros_ec_dev_driver definition
under the id_table member like all other platform drivers.

Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>