bhnd: another build fix for GCC-using architectures

Further to r299119. GCC architectures failed with
bcma_subr.c:138: warning: control reaches end of non-void function

Sponsored by:	The FreeBSD Foundation

sys/dev/bhnd/bcma/bcma_subr.c

@@ -134,6 +134,8 @@ bcma_corecfg_get_port_list(struct bcma_corecfg *cfg, bhnd_port_type type)
 	case BHND_PORT_AGENT:
 		return (&cfg->wrapper_ports);
 		break;
+	default:
+		return (NULL);
 	}
 }