hosted: fix ephemeral max frequency

[perigoso]

Detailed description

On BMDA, the scan routines set the interface frequencies according to command line options, always, meaning any changes by the user through monitor commands are overriden. This fixes that, this is a rather simplistic fix, that just resolves the issue, there is space for improvement on this code generally, but that would be more involved and can be done later.

This adds storage to the max frequency, meaning the frequency set by the user will be remembered over the initial command line value.

Before:

setting SWD frequency with mon freq 400000
immediately followed by a scan mon swd

Speed set to 0.400MHz for SWD
Speed set to 1.600MHz for SWD
-> jlink_swd_init(0)
Switching out of dormant state into SWD
jlink_swd_seq_out 32 clock_cycles: ffffffff

...

After:
setting SWD frequency with mon freq 400000
immediately followed by a scan mon swd

Speed set to 0.400MHz for SWD
Speed set to 0.400MHz for SWD
-> jlink_swd_init(0)
Switching out of dormant state into SWD
jlink_swd_seq_out 32 clock_cycles: ffffffff

...

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

fixes #1691

[dragonmux]

There are a couple of wording items we'd like to discuss that may need some change, but otherwise this looks great.

[dragonmux]

LGTM, we'll test this tomorrow and get it merged once we have.