Fixed derivable_impls lint that was breaking CI since 1.91

[Wassasin]

Since 1.91 was released we are getting the following error on CI:

Checking embassy-imxrt v0.1.0 (/home/runner/work/embassy-imxrt/embassy-imxrt)
error: this `impl` can be derived
   --> src/espi.rs:208:1
    |
208 | / impl Default for PortConfig {
209 | |     fn default() -> Self {
210 | |         Self::Unconfigured
211 | |     }
212 | | }
    | |_^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/rust-1.91.0/index.html#derivable_impls
    = note: `-D clippy::derivable-impls` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::derivable_impls)]`
help: replace the manual implementation with a derive attribute and mark the default variant
    |
102 + #[derive(Default)]
103 ~ pub enum PortConfig {
104 |     /// Unconfigured
105 ~     #[default]
106 ~     Unconfigured,
    |

This PR fixes this lint.