platform/chrome: fix memory corruption in ioctl

If "s_mem.bytes" is larger than the buffer size it leads to memory
corruption.

Fixes: eda2e30c ("mfd / platform: cros_ec: Miscellaneous character device to talk with the EC")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Link: https://lore.kernel.org/r/Yv8dpCFZJdbUT5ye@kili