Fix structure of SEC-SIGLOG

[Niederb]

This PR fixes the SEC-SIGLOG message type by moving the reserved bytes into the repeated group as suggested by the interface description (Firmware version 1.40 and 1.50):
https://content.u-blox.com/sites/default/files/documents/u-blox-F9-HPG-1.50_InterfaceDescription_UBXDOC-963802114-12815.pdf

Testing

I tested this change on a receiver with version 1.50

Checklist:

• I agree to abide by the code of conduct (see CODE_OF_CONDUCT.md).
• My code follows the style guidelines of this project (see CONTRIBUTING.MD).
• I have performed a self-review of my own code.
• (if appropriate) I have cited my u-blox documentation source(s).
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• (if appropriate) I have added test cases to the tests/test_*.py unittest suite to maintain >= 99% code coverage.
• I have tested my code against the full tests/test_*.py unittest suite.
• My changes generate no new warnings.
• Any dependent changes have been merged and published in downstream modules.
• I have signed my commits.
• I understand and acknowledge that the code will be published under a BSD 3-Clause license.

[semuadmin]

@Niederb - Good spot, thanks - will go into v1.2.45 shortly.