embassy-stm32/eth: convert LAN8742 driver to generic SMI driver

SMI Ethernet PHYs all share a common base set of registers that can do
90% of all tasks. The LAN8742 driver used some vendor-specific
registers to check link negotiation status, but the need for that was
debatable, so I migrated it to a generic driver instead, anybody who
wants extra functionality can copy it and impl their own on top of it.

embassy-stm32/src/eth/mod.rs

@@ -5,7 +5,7 @@
 #[cfg_attr(eth_v2, path = "v2/mod.rs")]
 #[cfg_attr(eth_v1, path = "v1.rs")]
 mod _version;
-pub mod lan8742a;
+pub mod generic_smi;
 
 pub use _version::*;