Dilemma workaround: reduce soft serial speed

[burkfers]

Description

The rp2040 half duplex serial implementation using the vendor driver is flawed. On an otherwise unmodified stock firmware with serial debugging enabled, somewhat frequent failed handshakes can be observed on the Dilemma v2 and Dilemma Max.

Until now, this had no observable effects in general use. The upcoming qmk breaking changes merge contains
qmk#21548, which exacerbates the issue and leads to spurious encoder events being generated, most often direction 0 for encoder 0, meaning a stock Dilemma will randomly scroll up.

Testing has revealed 38400 Bd to be stable with no observable downside on the Dilemma and Dilemma Max.

Credit/Thanks to:

• Drashna for leading me to the cause of the spurious encoder events I was observing, and linking past discussion on this problem
• Yingeling for reproducing on more hardware

Types of Changes

• Core
• Bugfix (rather: workaround)
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Issues Fixed or Closed by This PR

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).