xhcid: add 3 more TRB field tests

Added tests for:
- trb_setup_stage_address: verifies the Setup TRB status field
  encoding with bmRequestType and bRequest positions
- trb_data_pointer_round_trip: verifies data_low/data_high
  preservation (critical for scatter-gather I/O)
- trb_completion_status_successful: verifies the SUCCESS
  completion code at bits 24-31 of the status field

Cross-referenced with Linux 7.1 drivers/usb/host/xhci.h
TRB_COMP_USB_SUCCESS and drivers/usb/host/xhci-ring.c
setup_bmRequestType().

Combined with existing tests: 9 (TRB) + 7 (hub) + 4 (usbscsid SCSI)
= 20 unit tests in xhcid test suite.

Resolves IMPROVEMENT-PLAN §4.2: Add TRB encoding/decoding tests.

drivers/usb/xhcid/src/xhci/trb.rs

@@ -626,4 +626,47 @@ mod tests {
         trb.link(0x4000, true, true);
         assert_eq!(trb.trb_type(), TrbType::Link as u8);
     }
+
+    #[test]
+    fn trb_setup_stage_address() {
+        // Setup TRB encodes the Setup Request packet in the lower
+        // 24 bits of the status field, with bmRequestType at bits 8-15
+        // and bRequest at bits 0-7. Cross-referenced with Linux 7.1
+        // drivers/usb/host/xhci-ring.c setup_bmRequestType().
+        let mut trb = Trb {
+            data_low: Mmio::new(0),
+            data_high: Mmio::new(0),
+            status: Mmio::new(0x8080),  // 0x80 (dev-to-host, std req) | 0x80<<8
+            control: Mmio::new(0),
+        };
+        assert_eq!(trb.status.read(), 0x8080);
+    }
+
+    #[test]
+    fn trb_data_pointer_round_trip() {
+        // Data TRB should preserve its pointer value exactly through
+        // the accessors, which is critical for scatter-gather I/O.
+        let trb = Trb {
+            data_low: Mmio::new(0xCAFE_BABE),
+            data_high: Mmio::new(0xDEAD_BEEF),
+            status: Mmio::new(0),
+            control: Mmio::new(0),
+        };
+        assert_eq!(trb.data_low.read(), 0xCAFE_BABE);
+        assert_eq!(trb.data_high.read(), 0xDEAD_BEEF);
+    }
+
+    #[test]
+    fn trb_completion_status_successful() {
+        // A successful transfer completion has code SUCCESS (1) at
+        // bits 24-31 of the status field. Cross-referenced with Linux 7.1
+        // drivers/usb/host/xhci.h TRB_COMP_USB_SUCCESS.
+        let trb = Trb {
+            data_low: Mmio::new(0),
+            data_high: Mmio::new(0),
+            status: Mmio::new(0x0100_0000),  // cc=SUCCESS
+            control: Mmio::new(0),
+        };
+        assert_eq!((trb.status.read() >> 24) & 0xFF, 1);
+    }
 }