rtl8168d: log link state at end of init() via phys_sts register

The existing init() resets the chip, sets up descriptor rings, and
enables TX+RX. It does not read phys_sts, so a driver that comes
up on bare metal with the link not yet up (autoneg in progress,
no cable, or forced-power-down from the BIOS) silently
transmits into a dead medium.

Add a single phys_sts read at the end of init(). The Realtek
part numbers bits as: bit 0 = link up, bit 1 = 100 Mb/s,
bit 2 = 1000 Mb/s, bits 4-5 = duplex encoding. When the link
is not up, log a warn!() with the raw phys_sts byte so the
operator can correlate with the link partner. When the link
is up, log the detected speed as info!().

This is a real surface artefact (visible in the dmesg/log)
and does NOT introduce any new registers or commands; the
chip reports phys_sts on every read regardless of whether
the driver queries it.

The full PHY access (MII writes to PHY register 0x1F to read
the chip version, then BMCR soft-reset + autoneg advertisement)
remains [DEFERRED] until we have an MDIO read/write
infrastructure exposed in redox-driver-sys or linux-kpi
that works with the Redox MII bus scheme.

drivers/net/rtl8168d/src/device.rs

@@ -342,6 +342,31 @@ impl Rtl8168 {
         // Lock config
         self.regs.cmd_9346.write(0);
 
+        // Read physical link state. The Realtek part numbers its
+        // phys_sts bits: bit 0 = link up, bit 1 = speed-100,
+        // bit 2 = speed-1000, bits 4-5 encode the duplex. This is
+        // a read-only check; a failed link is not an error but
+        // produces a warning that helps diagnose a missing
+        // autonegotiation on bare metal. The driver continues to
+        // emit TX descriptors regardless; if no cable is present
+        // the peer simply does not reply.
+        let phys_sts = self.regs.phys_sts.read();
+        if phys_sts & 0x01 == 0 {
+            log::warn!(
+                "rtl8168d: link is down at start; check cable / \
+                 autoneg status (phys_sts = 0x{:02x})",
+                phys_sts
+            );
+        } else {
+            let mbps = match (phys_sts >> 4) & 0x03 {
+                0b00 => 10,
+                0b01 => 100,
+                0b10 => 1000,
+                _ => 0,
+            };
+            log::info!("rtl8168d: link up at {} Mb/s", mbps);
+        }
+
         log::debug!("Complete!");
         Ok(())
     }