usbsdmux: sd_regs: Fix ruff error regarding the use of percent formatting

[hnez]

It looks like ruff has learned a new trick, making the CI pipeline in #88 fail, even though it did not touch the python code at all.
This PR should fix that.

The primary goal of this commit is fixing the following ruff linting error:

usbsdmux/sd_regs.py:39:50: UP031 Use format specifiers instead of percent format
   |
37 |             text.append(f"  {f['field']}")
38 |         raw = f["raw"]
39 |         text.append(f"    raw: 0b{format(raw[0], '0%db' % raw[1])} == 0x{raw[0]:0x} == {int(raw[0])}")
   |                                                  ^^^^^^ UP031
40 |         if "enum" in f:
41 |             text.append(f"    enum: {f['enum']} {f.get('unit', '')}".rstrip())
   |
   = help: Replace with format specifiers

While at it I had a hard time understanding what the offending code does, so I have decided to restructure it a bit while at it.

The code path is covered by the pytest testsuite, so we can be somewhat certain that the behaviour did not change.