Several refactors and improvements for `net_bytes_from_str` as follows:
- Replaced manual hex digit checks with `isxdigit()`.
- Changed variable `i` from unsigned int to size_t for consistency with
the `strlen()` return type.
- Added `src_len` to store the result of `strlen(src)` to avoid
multiple calls to `strlen` in the `for-loop`.
- Ensured casting to `unsigned char` before passing to `isxdigit()` to
prevent undefined behavior.
- Explicitly cast the result of `strtol()` to `uint8_t` to match
the buffer type.
Signed-off-by: Pisit Sawangvonganan <pisit@ndrsolution.com>
b34349eed9