leds: leds-cros_ec: Avoid -Wflex-array-member-not-at-end warning

-Wflex-array-member-not-at-end was introduced in GCC-14, and we are
getting ready to enable it, globally.

Replace an on-stack definition of a flexible structure with a call
to utility function cros_ec_cmd().

So, with these changes, fix the following warning:

drivers/leds/leds-cros_ec.c:70:40: 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>
Acked-by: Thomas Weißschuh <linux@weissschuh.net>
Link: https://lore.kernel.org/r/Z-rKcgFjsyKvd58q@kspp


Signed-off-by: Lee Jones <lee@kernel.org>