[ethernet] Fix Starting/Stopped/Starting log on RP2040 boot

Don't set started_ = true in setup(). Let the link polling in
loop() set it when the W5500 PHY link is actually up. Previously,
setting it immediately caused a spurious state transition because
the W5500 needs time after begin() before the link is ready.

esphome/components/ethernet/ethernet_component_rp2040.cpp

@@ -75,8 +75,10 @@ void EthernetComponent::setup() {
   // via __addEthernetPacketHandler when no interrupt pin is used,
   // or via GPIO interrupt when one is provided.
 
-  // Mark as started - connection will be detected in loop()
-  this->started_ = true;
+  // Don't set started_ here — let the link polling in loop() set it
+  // when the W5500 link is actually up. Setting it prematurely causes
+  // a "Starting → Stopped → Starting" log sequence because the W5500
+  // needs time after begin() before the PHY link is ready.
 }
 
 void EthernetComponent::loop() {