iwlwifi: do not use __unused as variable name

On various systems __unused is a define to __attribute__((__unused__))
and yields compile errors as a result. Use __spare instead to describe
that these bits are "unused".

Sponsored by:  The FreeBSD Foundation
Signed-off-by: Bjoern A. Zeeb <bz@FreeBSD.ORG>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>

drivers/net/wireless/intel/iwlwifi/iwl-fh.h

@@ -580,7 +580,7 @@ struct iwl_rb_status {
 	__le16 closed_fr_num;
 	__le16 finished_rb_num;
 	__le16 finished_fr_nam;
-	__le32 __unused;
+	__le32 __spare;
 } __packed;