iio: proximity: cros_ec_mkbp_proximity: 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.

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/iio/proximity/cros_ec_mkbp_proximity.c:63:40: 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]>
Reviewed-by: Tzung-Bi Shih <[e-mail]>
Link: https://patch.msgid.link/Z9lEYdPPIyiRGm36@kspp
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/proximity/cros_ec_mkbp_proximity.c

···
 static int cros_ec_mkbp_proximity_query(struct cros_ec_device *ec_dev,
 					int *state)
 {
-	struct {
-		struct cros_ec_command msg;
-		union {
-			struct ec_params_mkbp_info params;
-			u32 switches;
-		};
-	} __packed buf = { };
-	struct ec_params_mkbp_info *params = &buf.params;
-	struct cros_ec_command *msg = &buf.msg;
-	u32 *switches = &buf.switches;
+	DEFINE_RAW_FLEX(struct cros_ec_command, buf, data,
+			MAX(sizeof(u32), sizeof(struct ec_params_mkbp_info)));
+	struct ec_params_mkbp_info *params = (struct ec_params_mkbp_info *)buf->data;
+	struct cros_ec_command *msg = buf;
+	u32 *switches = (u32 *)buf->data;
 	size_t insize = sizeof(*switches);
 	int ret;