thunderbolt: Fix INVARIANTS compilation

The problem is that THUNDERBOLT_DEBUG triggers the use of some
variables, but it is independent of INVARIANTS and the variables it uses
were tagged with '__diagused'.

Fix this by using '__maybe_unused' in those places.

Fixes:          1836330791 ("thunderbolt: make code -Wunused clean")
Fixes:          886164895f ("amd64: complete thunderbolt KERNCONF integration")
Sponsored by:   The FreeBSD Foundation

sys/dev/thunderbolt/nhi.c

@@ -834,7 +834,7 @@ static int
 nhi_tx_complete(struct nhi_ring_pair *r, struct nhi_tx_buffer_desc *desc,
     struct nhi_cmd_frame *cmd)
 {
-	struct nhi_softc *sc __diagused;
+	struct nhi_softc *sc __maybe_unused;
 	struct nhi_pdf_dispatch *txpdf;
 	u_int sof;
 
@@ -868,10 +868,10 @@ static int
 nhi_rx_complete(struct nhi_ring_pair *r, struct nhi_rx_post_desc *desc,
     struct nhi_cmd_frame *cmd)
 {
-	struct nhi_softc *sc __diagused;
+	struct nhi_softc *sc __maybe_unused;
 	struct nhi_pdf_dispatch *rxpdf;
 	u_int eof;
-	u_int len __diagused;
+	u_int len __maybe_unused;
 
 	sc = r->sc;
 	eof = desc->eof_len >> RX_BUFFER_DESC_EOF_SHIFT;

sys/dev/thunderbolt/router.c

@@ -278,7 +278,7 @@ _tb_router_attach(struct router_softc *sc)
 	struct tb_cfg_router *cfg;
 	uint32_t *buf;
 	int error;
-	int up __diagused;
+	int up __maybe_unused;
 
 	buf = malloc(9 * 4, M_THUNDERBOLT, M_NOWAIT|M_ZERO);
 	if (buf == NULL)
@@ -720,7 +720,7 @@ router_notify_intr(void *context, union nhi_ring_desc *ring, struct nhi_cmd_fram
 	struct router_softc *sc;
 	struct router_command *cmd;
 	struct tb_cfg_notify event;
-	u_int adap __diagused;
+	u_int adap __maybe_unused;
 	u_int ev;
 
 	KASSERT(context != NULL, ("context cannot be NULL\n"));

sys/dev/thunderbolt/tb_acpi_pcib.c

@@ -120,7 +120,7 @@ tb_acpi_pcib_attach(device_t dev)
 		ACPI_OBJECT_LIST list;
 		ACPI_OBJECT arg;
 		ACPI_BUFFER buf;
-		ACPI_STATUS s __diagused;
+		ACPI_STATUS s __maybe_unused;
 
 		tb_debug(sc, DBG_BRIDGE, "Executing OSUP\n");
 

sys/dev/thunderbolt/tb_pcib.c

@@ -304,7 +304,7 @@ static int
 tb_pcib_detach(device_t dev)
 {
 	struct tb_pcib_softc *sc;
-	int error __diagused;
+	int error __maybe_unused;
 
 	sc = device_get_softc(dev);