platform/chrome: cros_ec_proto: remove big stub objects from stack

sizeof(struct device) = 680
sizeof(struct cros_ec_dev) = 720

They tend to exceed the stack frame size limit in some specific
environment which results in the following compilation error:

>> drivers/platform/chrome/cros_ec_proto_test.c:2530:13: error: stack
frame size (2128) exceeds limit (2048) in
'cros_ec_proto_test_get_sensor_count_legacy'

Remove the big stub objects from stack.

This is:
$ sed -i 's/struct cros_ec_dev /static struct cros_ec_dev /' \
    drivers/platform/chrome/cros_ec_proto_test.c

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20230117080254.2725536-1-tzungbi@kernel.org