mfd: cros_ec: Prevent data transfer while device is suspended

The cros_ec driver is still active while the device is suspended.
Besides that, it also tries to transfer data even after the I2C host had
been suspended. This patch uses a simple flag to prevent this.

Signed-off-by: Joseph Lo <josephl@nvidia.com>
Signed-off-by: Thierry Escande <thierry.escande@collabora.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>