fix(PeriphDrivers): Fix `MXC_SYS_GetUSN` Buffer Overflow

[Jake-Carter]

Description

Fixes #1006

This PR updates MXC_SYS_GetUSN to prevent buffer overflow. The application-side buffer should be MXC_SYS_USN_LEN, but we were using MXC_SYS_USN_CHECKSUM_LEN which is a larger buffer size with some additional padding used for the internal CRC calculation.

Checklist Before Requesting Review

• [ ] PR Title follows correct guidelines.
• [ ] Description of changes and all other relevant information.
• [ ] (Optional) Link any related GitHub issues using a keyword
• [ ] (Optional) Provide info on any relevant functional testing/validation. For API changes or significant features, this is not optional.

[EricB-ADI]

Can we get this reviewed and merge this in?

[Jake-Carter]

Can we get this reviewed and merge this in?

Yep, ready for review