[ethernet] Address additional review feedback

- Remove dead ENC28J60 entry from _IDF6_ETHERNET_COMPONENTS
- Simplify get_duplex_mode() — both chips are full-duplex on RP2040
- Add #error guard for unsupported RP2040 SPI Ethernet type

esphome/components/ethernet/__init__.py

@@ -157,7 +157,6 @@ _IDF6_ETHERNET_COMPONENTS: dict[str, IDFRegistryComponent] = {
     "KSZ8081RNA": IDFRegistryComponent("espressif/ksz80xx", "1.0.0"),
     "W5500": IDFRegistryComponent("espressif/w5500", "1.0.1"),
     "DM9051": IDFRegistryComponent("espressif/dm9051", "1.0.0"),
-    "ENC28J60": IDFRegistryComponent("espressif/enc28j60", "1.0.1"),
 }
 
 SPI_ETHERNET_TYPES = ["W5500", "DM9051", "ENC28J60"]

esphome/components/ethernet/ethernet_component.h

@@ -27,6 +27,8 @@ extern "C" eth_esp32_emac_config_t eth_esp32_emac_default_config(void);
 #include <W5500lwIP.h>
 #elif defined(USE_ETHERNET_ENC28J60)
 #include <ENC28J60lwIP.h>
+#else
+#error "Unsupported RP2040 SPI Ethernet type"
 #endif
 #endif
 
@@ -224,6 +226,8 @@ class EthernetComponent final : public Component {
   Wiznet5500lwIP *eth_{nullptr};
 #elif defined(USE_ETHERNET_ENC28J60)
   ENC28J60lwIP *eth_{nullptr};
+#else
+#error "Unsupported RP2040 SPI Ethernet type"
 #endif
   uint32_t last_link_check_{0};
   uint8_t clk_pin_;

esphome/components/ethernet/ethernet_component_rp2040.cpp

@@ -226,13 +226,8 @@ const char *EthernetComponent::get_eth_mac_address_pretty_into_buffer(
 }
 
 eth_duplex_t EthernetComponent::get_duplex_mode() {
-#ifdef USE_ETHERNET_ENC28J60
-  // ENC28J60 is configured for full-duplex by the arduino-pico driver
+  // Both W5500 and ENC28J60 are full-duplex on RP2040
   return ETH_DUPLEX_FULL;
-#else
-  // W5500 is always full duplex
-  return ETH_DUPLEX_FULL;
-#endif
 }
 
 eth_speed_t EthernetComponent::get_link_speed() {