staging: r8188eu: clean up on_action_public

Use the struct mgmt to read the action_code. This is much simpler than
parsing the message ourselves.

Add a comment about reading the action code. All members of the action
enum start with an action_code byte. It does not matter which member we
use.

Tested-by: Philipp Hortmann <philipp.g.hortmann@gmail.com> # Edimax N150
Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20221030173326.1588647-10-martin@kaiser.cxSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/r8188eu/core/rtw_mlme_ext.c

@@ -3813,23 +3813,16 @@ static unsigned int on_action_public_default(struct recv_frame *precv_frame)
 static void on_action_public(struct adapter *padapter, struct recv_frame *precv_frame)
 {
 	struct ieee80211_mgmt *mgmt = (struct ieee80211_mgmt *)precv_frame->rx_data;
-	u8 *pframe = precv_frame->rx_data;
-	u8 *frame_body = pframe + sizeof(struct ieee80211_hdr_3addr);
-	u8 action;
 
 	/* check RA matches or not */
 	if (memcmp(myid(&padapter->eeprompriv), mgmt->da, ETH_ALEN))
 		return;
 
-	action = frame_body[1];
-	switch (action) {
-	case ACT_PUBLIC_VENDOR:
+	/* All members of the action enum start with action_code. */
+	if (mgmt->u.action.u.s1g.action_code == WLAN_PUB_ACTION_VENDOR_SPECIFIC)
 		on_action_public_vendor(precv_frame);
-		break;
-	default:
+	else
 		on_action_public_default(precv_frame);
-		break;
-	}
 }
 
 static void OnAction_p2p(struct adapter *padapter, struct recv_frame *precv_frame)