fix(eth): Set default clock in pin value to zero

[me-no-dev]

When Arduino is used as component, CONFIG_ETH_RMII_CLK_IN_GPIO might not be defined, so we set it to const 0 to clear the issue.

Fixes: #10440

[github-actions]

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[github-actions]

Memory usage test (comparing PR against master branch)

The table below shows the summary of memory usage change (decrease - increase) in bytes and percentage for each target.

Memory	FLASH [bytes]		FLASH [%]		RAM [bytes]		RAM [%]
Target	DEC	INC	DEC	INC	DEC	INC	DEC	INC
ESP32P4	0	0	0.00	0.00	0	0	0.00	0.00
ESP32S3	0	‼️ +17K	0.00	‼️ +2.68	0	‼️ +2K	0.00	‼️ +8.27
ESP32S2	0	‼️ +14K	0.00	‼️ +2.26	0	‼️ +2K	0.00	‼️ +8.80
ESP32C3	0	‼️ +20K	0.00	‼️ +3.26	0	‼️ +2K	0.00	‼️ +10.32
ESP32C6	0	‼️ +35K	0.00	‼️ +3.88	0	‼️ +22K	0.00	‼️ +127.95
ESP32H2	0	‼️ +11K	0.00	‼️ +2.07	0	‼️ +22K	0.00	‼️ +126.07
ESP32	0	‼️ +14K	0.00	‼️ +2.08	0	⚠️ +1368	0.00	‼️ +3.19

Click to expand the detailed deltas report [usage change in BYTES]

Target	ESP32P4		ESP32S3		ESP32S2		ESP32C3		ESP32C6		ESP32H2		ESP32
Example	FLASH	RAM	FLASH	RAM	FLASH	RAM	FLASH	RAM	FLASH	RAM	FLASH	RAM	FLASH	RAM
Ethernet/examples/ETH_TLK110	-	-	-	-	-	-	-	-	-	-	-	-	‼️ +10K	⚠️ +800
Ethernet/examples/ETH_W5500_Arduino_SPI	-	-	‼️ +14K	⚠️ +1940	‼️ +12K	⚠️ +1668	‼️ +17K	⚠️ +1744	‼️ +19K	‼️ +22K	‼️ +11K	‼️ +22K	‼️ +8K	⚠️ +736
Ethernet/examples/ETH_W5500_IDF_SPI	-	-	‼️ +14K	⚠️ +1908	‼️ +11K	⚠️ +1620	‼️ +17K	⚠️ +1744	‼️ +19K	‼️ +22K	‼️ +11K	‼️ +22K	‼️ +7K	⚠️ +672
Ethernet/examples/ETH_WIFI_BRIDGE	-	-	‼️ +17K	‼️ +2K	‼️ +14K	‼️ +2K	‼️ +20K	‼️ +2K	‼️ +35K	‼️ +4K	-	-	‼️ +14K	⚠️ +1368
Ethernet/examples/ETH_LAN8720	-	-	-	-	-	-	-	-	-	-	-	-	‼️ +10K	⚠️ +800