smoltcp::Device implementation can be for EthernetDMA instead of &mut EthernetDMA

[datdenkikniet]

The current implementation still has the following signature:

impl<'a, 'rx, 'tx> smoltcp::Device for &'a mut EthernetDMA<'rx, 'tx> {}

However, the 'a lifetime is completely unused. We could change the impl to

impl<'rx, 'tx> smoltcp::Device for EthernetDMA<'rx, 'tx> {}

We should investigate whether this is still necessary (unlikely), or if it is just an artifact from how the Device trait worked in smoltcp 0.8 and older (likely).

Removing that lifetime and impl-ing directly for EthernetDMA would also remove the current oddness w.r.t. requiring that an &mut &mut EthernetDMA is passed to smoltcp functions.