bluetooth: shell: avoid multiple strlen calls

[ndrs-pst]

Add len to store the result of strlen(addr_arg) to avoid multiple calls to strlen within the for-loop in cmd_scan_filter_set_addr.
While the performance impact may be minimal in a shell context, storing strlen(addr_arg) in len ensures a single call, making the code more predictable and consistent.

[alwa-nordic]

Looks good. Just a comment on the commit message: The commit message does not say why it's good to avoid multiple calls to strlen. Can you give your reason? Is it performance, or readability? I assume it's the latter. But it's better when the contributor documents the thinking that went into a change. :)

[ndrs-pst]

Thank you for the feedback!
I'll update the commit message to highlight the consistency aspect.
While runtime impact may be minimal in a shell context, this pattern appears in several places in the Zephyr codebase, with an assumption that strlen would only be called once and stored optimistically by the compiler. By storing it in len, we ensure a single strlen call, making the code more predictable.

[ndrs-pst]

@alwa-nordic, Oh BTW, it looks like @Thalley wants some feedback from you on this PR #74652 😃