Bluetooth: Host: Use macro for nanoseconds per second

[alexapostolu]

Use the macro NSEC_PER_SEC defined in sys_clock.h for nanoseconds per second when updating write stats.

Fixes: #62641

Issue #62641 was mostly implemented, but as a comment pointed out there were two files that still used the 1000000000U constant instead of the macro NSEC_PER_SEC, which is defined as ((NSEC_PER_USEC) * (USEC_PER_MSEC) * (MSEC_PER_SEC)).

[github-actions]

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[jhedberg]

@alexapostolu thanks for the PR! You still need to make the Compliance check pass before this can be merged, however. Seems like you have a too long line in the commit message.

[Thalley]

Changes look good, but please fix compliance :)

[alexapostolu]

Fixed the commit message

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[github-actions]

Hi @alexapostolu!
Congratulations on getting your very first Zephyr pull request merged 🎉🥳. This is a fantastic achievement, and we're thrilled to have you as part of our community!

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Thank you for your valuable input, and we look forward to seeing more of your contributions in the future! 🪁