Fix: timer setup Flash usage

[dragonmux]

Detailed description

ALTracer was kind enough to point out on Discord that the timer setup code for BMP introduced in #1615 also wound up introducing ~400 bytes of extra Flash usage for a single function - timer_set_oc_mode() - so this PR, in combination with some changes to libopencm3, addresses this.

This results in a 332 byte saving.

Your checklist for this pull request

• I've read the Code of Conduct
• I've read the guidelines for contributing to this repository
• It builds for hardware native (make PROBE_HOST=native)
• It builds as BMDA (make PROBE_HOST=hosted)
• I've tested it to the best of my ability
• My commit messages provide a useful short description of what the commits do

Closing issues

[esden]

Good catch. Thanks for fixing it. :)