fix: unit mismatch in firmware backup space check

[joshschmelzle]

Type of change

• Bug fix (non-breaking change that fixes something)
• Documentation update (readme, changelog, man page, etc.)
• New feature (non-breaking change adding functionality)
• Breaking change (fix or feature changing existing functionality)
• Code quality improvement (refactor, performance improvements)
• Other (please specify):

Breaking change

• What functionality breaks?
• Why does it break?
• How should it be migrated?
• Are there any backward-compatible alternatives?

Proposed change

The check_space() function was comparing bytes (from stat) against
kilobytes (from df), causing false "insufficient space" errors.

Testing

• Did you add new automated tests? If yes, explain which edge cases are covered.
• Does this change affect any existing tests? If so, how did you adjust them?
• Please provide test run results or screenshots if applicable.

Before change:

wlanpi@wlanpi-fba:~ $ wlanpi-iwlwifi-ucode-92 disable
[2025-09-19 11:17:41] Creating original backup...
[2025-09-19 11:17:41] ERROR: Insufficient space for backup ...

After change:

wlanpi@wlanpi-fba:~ $ wlanpi-iwlwifi-ucode-92 disable
[2025-09-19 11:22:52] Creating original backup...
[2025-09-19 11:22:52] Original backup created successfully ...
[2025-09-19 11:22:52] Moving firmware file to disabled state ...
[2025-09-19 11:22:52] Firmware disabled ...
[2025-09-19 11:22:52] Resetting wireless modules ...
[2025-09-19 11:22:55] Wireless modules reset complete ...

Checklist

• I have read the contribution guidelines and policies
• I have targeted this PR against the correct git branch (this can vary depending on the default branch of the repo)
• I ran the test suite and verified it succeeded
• I linked a GitHub issue to this PR (in the next section).
• I have updated the changelog (if applicable)
• I have added or updated the documentation (if applicable)

Related Issues/PRs

• This PR fixes/closes issue [BUG]: wlanpi-iwlwifi-ucode-92.sh script not working on the Go #51
• This PR is related to issue #
• This PR depends on/blocks PR #