core/identity/ecdsa: fix clippy::op-ref, clippy::needless-borrow

[elenaf9]

Description

Fix clippy::op-ref, clippy::needless-borrow, which cause an error if you run cargo custom-clippy directly in the /core package or depend on it. See https://github.com/kpp/rust-libp2p/runs/7489162901?check_suite_focus=true.

Does this need a changelog entry?

[thomaseizinger]

No changelog entry needed IMO :)

But curious why this isn't flagged by CI?

[elenaf9]

But curious why this isn't flagged by CI?

I was also confused. The same happened in #2737. Took me actually a while to figure it out, though in hindsight the answer is rather obvious:
Our custom-clippy just runs clippy in the root package with default features, and in our root Cargo.toml we disable default features for dependencies like libp2p-core. Since ecdsa is behind a features flag it was not checked.
I think it would make more sense to check all packages with all features. Opened #2771.

[mxinden]

Thanks for the fix and thanks for looking into why this wasn't caught before.

Also don't think this needs a changelog.