wifi: remove zero-length arrays

All of these are really meant to be variable-length, and
in the case of s1g_beacon it's actually accessed. Make that
one in particular, and a couple of others (that aren't used
as arrays now), actually variable.

Reported-by: [e-mail]
Fixes: 1e1f706fc2ce ("wifi: cfg80211/mac80211: correctly parse S1G beacon optional elements")
Link: https://patch.msgid.link/20250614003037.a3e82e882251.I2e8b58e56ff2a9f8b06c66f036578b7c1d4e4685@changeid
Signed-off-by: Johannes Berg <[e-mail]>

include/linux/ieee80211.h

···
 			u8 sa[ETH_ALEN];
 			__le32 timestamp;
 			u8 change_seq;
-			u8 variable[0];
+			u8 variable[];
 		} __packed s1g_beacon;
 	} u;
 } __packed __aligned(2);
···
 					u8 action_code;
 					u8 dialog_token;
 					__le16 capability;
-					u8 variable[0];
+					u8 variable[];
 				} __packed tdls_discover_resp;
 				struct {
 					u8 action_code;
···
 		struct {
 			u8 dialog_token;
 			__le16 capability;
-			u8 variable[0];
+			u8 variable[];
 		} __packed setup_req;
 		struct {
 			__le16 status_code;
 			u8 dialog_token;
 			__le16 capability;
-			u8 variable[0];
+			u8 variable[];
 		} __packed setup_resp;
 		struct {
 			__le16 status_code;
 			u8 dialog_token;
-			u8 variable[0];
+			u8 variable[];
 		} __packed setup_cfm;
 		struct {
 			__le16 reason_code;
-			u8 variable[0];
+			u8 variable[];
 		} __packed teardown;
 		struct {
 			u8 dialog_token;
-			u8 variable[0];
+			u8 variable[];
 		} __packed discover_req;
 		struct {
 			u8 target_channel;
 			u8 oper_class;
-			u8 variable[0];
+			u8 variable[];
 		} __packed chan_switch_req;
 		struct {
 			__le16 status_code;
-			u8 variable[0];
+			u8 variable[];
 		} __packed chan_switch_resp;
 	} u;
 } __packed;