platform/chrome: cros_ec_proto: avoid -Wflex-array-member-not-at-end warnings

platform/chrome: cros_ec_proto: avoid -Wflex-array-member-not-at-end warnings

Use the `DEFINE_RAW_FLEX()` helper for an on-stack definition of
a flexible structure where the size of the flexible-array member
is known at compile-time, and refactor the rest of the code,
accordingly.

So, with these changes, fix the following warning:
drivers/platform/chrome/cros_ec_proto_test.c:1547:40: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
drivers/platform/chrome/cros_ec_proto_test.c:1607:40: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
drivers/platform/chrome/cros_ec_proto_test.c:1645:40: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
drivers/platform/chrome/cros_ec_proto_test.c:1668:40: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

Link: https://github.com/KSPP/linux/issues/202
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/ZgMaDl/of8YC445S@neat
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>