bluetooth: shell: avoid multiple `strlen` calls

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.

Signed-off-by: Pisit Sawangvonganan <pisit@ndrsolution.com>