Fix: s32k3xx: Remove the single assert

[ALTracer]

Detailed description

• No new features.
• The existing problem is that the S32K3xx driver, introduced in Add debug and flash support for S32K344 #1652, references assert() which pulls fiprintf() and _vfiprintf_r again, resulting in extra 1.5 KiB of newlib stdio.
• This PR solves it by replacing the assert with a logged runtime error. IIUC target flash layer should call it with a len=writesize. Depending on review and API guarantees I can drop even that. Update: dropped.

I expect a significant size-diff from CI. This should restore the effect of my previous #1513.
Cannot test on hardware. CC @via.

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 (see Building the firmware)
• It builds as BMDA (see Building the BMDA)
• 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

[dragonmux]

Given the target Flash API layer guarantees this property already - LGTM, merging once the builds complete. Thank you for the contribution!