Fix TN9710P probe on MV88X3310 boards

[Skyline-23]

Summary

• treat a zero PHY ID as the actual no-PHY signal so MV88X3310 boards on MDIO port 0 can probe
• reset the PHY on the scanned MDIO port instead of hardcoding port 1
• pass the DMA address directly to dma_free_coherent() to avoid the probe-time crash seen on Ubuntu 24.04 / kernel 6.8

Validation

• reproduced on a Tehuti TN9710P card (1fc9:4027, subsystem 1fc9:3015)
• before this change, the driver failed with PHY init failed
• with the MDIO fix plus the MV88X3310 firmware blob present, the driver detects the PHY and creates enp3s0
• before the DMA fix, probe then crashed in bdx_tx_free(); after the fix, the crash no longer reproduces

Notes

• the MV88X3310 firmware blob is still required for this board, but is not included here