rtc: cros-ec: Avoid a couple of -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/rtc/rtc-cros-ec.c:62:40: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
drivers/rtc/rtc-cros-ec.c:40:40: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Tzung-Bi Shih <tzungbi@kernel.org>
Link: https://lore.kernel.org/r/Z9PpPg06OK8ghNvm@kspp


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>