add 0x8b, lifted from Linux iwlegacy/commands.h

This is "STA invalid". I saw it during some 4965 testing (kern/183260)
and I still have no idea what is causing it.

Obtained from:	Linux drivers/net/wireless/iwlegacy

sys/dev/iwn/if_iwnreg.h

@@ -1234,6 +1234,7 @@ struct iwn_ucode_info {
 #define IWN_TX_FAIL_FIFO_UNDERRRUN	0x84	/* tx fifo not kept running */
 #define IWN_TX_FAIL_DEST_IN_PS		0x88	/* sta found in power save */
 #define IWN_TX_FAIL_TX_LOCKED		0x90	/* waiting to see traffic */
+#define IWN_TX_FAIL_STA_INVALID		0x8b	/* XXX STA invalid (???) */
 
 struct iwn4965_tx_stat {
 	uint8_t		nframes;