cros_ec: sandbox: Use correct value for number of slots

cros_ec: sandbox: Use correct value for number of slots

In the definition of struct ec_state the number of slots that are
created is VSTORE_SLOT_COUNT (==4) but the value of req->slot is
checked against EC_VSTORE_SLOT_MAX (==32) so this can lead to memory
access beyond that allocated.
Instead change the size check to use VSTORE_SLOT_COUNT to ensure it
matches what has actually been allocated.

This issue found by Smatch.

Signed-off-by: Andrew Goodbody <andrew.goodbody@linaro.org>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>