Changed attribute by value passing to by pointer to const.

[wdl-crestron]

See #67

[pabs3]

@wdl-crestron Thanks for the contribution. Since this patch series changes the public headers, probably it also changes the ABI too, which would need the library SONAME to be bumped. So please verify if the ABI is changed using abi-compliance-checker etc or libabigail and if that is needed, then include an ABI change. https://sourceware.org/libabigail/ https://github.com/lvc/abi-compliance-checker Since the public headers are changed, probably the gt tool needs to be updated to use those changes too, if so, please submit a PR for that: https://github.com/linux-usb-gadgets/gt Also some polish is needed: * Please include explanations of the changes in each commit message. * Please move the whitespace cleanup change to another PR. * Please use tabs instead of spaces for indentation of new code. * Please use put the URL to fixed issues in a Fixes commit msg footer.

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-- bye, pabs https://bonedaddy.net/pabs3/

[wdl-crestron]

Yes ABI changed. This is report summary from abi-compliance-checker: