platform/chrome: cros_kbd_led_backlight: Avoid -Wflex-array-member-not-at-end warnings

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

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 warnings:

drivers/platform/chrome/cros_kbd_led_backlight.c:141:40: 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_kbd_led_backlight.c:162:40: 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 <[e-mail]>
Link: https://lore.kernel.org/r/Z-afGnRbyGs4dHb1@kspp
Signed-off-by: Tzung-Bi Shih <[e-mail]>

drivers/platform/chrome/cros_kbd_led_backlight.c

···
 keyboard_led_set_brightness_ec_pwm(struct led_classdev *cdev,
 				   enum led_brightness brightness)
 {
-	struct {
-		struct cros_ec_command msg;
-		struct ec_params_pwm_set_keyboard_backlight params;
-	} __packed buf;
-	struct ec_params_pwm_set_keyboard_backlight *params = &buf.params;
-	struct cros_ec_command *msg = &buf.msg;
+	DEFINE_RAW_FLEX(struct cros_ec_command, msg, data,
+			sizeof(struct ec_params_pwm_set_keyboard_backlight));
+	struct ec_params_pwm_set_keyboard_backlight *params =
+			(struct ec_params_pwm_set_keyboard_backlight *)msg->data;
 	struct keyboard_led *keyboard_led = container_of(cdev, struct keyboard_led, cdev);
-
-	memset(&buf, 0, sizeof(buf));
 
 	msg->command = EC_CMD_PWM_SET_KEYBOARD_BACKLIGHT;
 	msg->outsize = sizeof(*params);
···
 static enum led_brightness
 keyboard_led_get_brightness_ec_pwm(struct led_classdev *cdev)
 {
-	struct {
-		struct cros_ec_command msg;
-		struct ec_response_pwm_get_keyboard_backlight resp;
-	} __packed buf;
-	struct ec_response_pwm_get_keyboard_backlight *resp = &buf.resp;
-	struct cros_ec_command *msg = &buf.msg;
+	DEFINE_RAW_FLEX(struct cros_ec_command, msg, data,
+			sizeof(struct ec_response_pwm_get_keyboard_backlight));
+	struct ec_response_pwm_get_keyboard_backlight *resp =
+			(struct ec_response_pwm_get_keyboard_backlight *)msg->data;
 	struct keyboard_led *keyboard_led = container_of(cdev, struct keyboard_led, cdev);
 	int ret;
-
-	memset(&buf, 0, sizeof(buf));
 
 	msg->command = EC_CMD_PWM_GET_KEYBOARD_BACKLIGHT;
 	msg->insize = sizeof(*resp);