fw: Fix erroneous type in frame header

[dehanj]

Description

The frame_header has the wrong type in the struct. It is not used as the enum, it is used for the actual length.

This does note seem to change the actual binary output, but I would still argue it should be the type size_t.
As it is in tkey-libs.

Type of change

Please tick any that are relevant to this PR and remove any that aren't.

• Bugfix (non breaking change which resolve an issue)

Submission checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my changes
• I have tested and verified my changes on target
• My changes are well written and CI is passing
• I have squashed my work to relevant commits and rebased on main for linear history
• I have updated the documentation where relevant (readme, dev.tillitis.se etc.)