Fix some more clippy lints

Hi! Thank you for helping out with SSD1306 development! Please:

[ ] Check that you've added documentation to any new methods
[x] Rebase from master if you're not already up to date
[ ] Add or modify an example if there are changes to the public API
[ ] Add a CHANGELOG.md entry in the Unreleased section under the appropriate heading (Added, Fixed, Changed, etc)
[x] Run rustfmt on the project with cargo fmt --all - CI will not pass without this step
[x] Check that your branch is up to date with master and that CI is passing once the PR is opened

PR description

Signed-off-by: Daniel Egger daniel@eggers-club.de