Fix JTAG sanity check in BYPASS device counting

[ALTracer]

Detailed description

• This is not a new feature.
• The existing problem is an error triggered in BMD logic for chips which are debuggable by OpenOCD using same adapteres.
• The PR solves it by ensuring DR value is predictable.

jtag_scan: Sanity check failed: BYPASS dev count doesn't match IR scan
Not tested yet. The error/warning demotion will be rebased away once I confirm the solution works.
Upd: tested to improve the situation, the chip in question became scannable by BMF. Sanity check restored.

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

[ALTracer]

Logs with a FT2232H-attached board (unsupported) on this branch. Please state whether I should keep or discard the jtag_scan: DR=1 by default in BYPASS message, which confirms such chips exist.

$ ./build/blackmagic -tjv 1 -c ftdijtag
Black Magic Debug App v2.0.0-304-g11bd1f2c
 for Black Magic Probe, ST-Link v2 and v3, CMSIS-DAP, J-Link, FTDI (MPSSE) and WCH-Link
Using 0403:6010 <no serial number> FTDI
 Dual RS232-HS ---
Running in Test Mode
Target voltage: Unknown
Speed set to 3.750MHz for JTAG
Resetting TAP
Change state to Shift-DR
Scanning out ID codes
Return to Run-Test/Idle
Change state to Shift-IR
Scanning out IRs
Return to Run-Test/Idle
Change state to Shift-DR
jtag_scan: DR=1 by default in BYPASS
Return to Run-Test/Idle
ID code 0x000006df: Unknown
Given target number 1 not available max 0

[dragonmux]

You've been able to prove the purpose of this PR with that message but we don't think it adds anything to scans more generally, so please drop the extra logging message.

[dragonmux]

LGTM, merging - thank you for this fix to the BYPASS check so it's more correct and doesn't kick out devices that start with their DR BYPASS bit set to 1 after reset or previous scan.