Bluetooth: Classic: HF_AG: Fix unexpected return

When `buf->len` is 0, the function of the while-loop
will be returned with error code `-ENOTSUP`.
The code block after while-loop cannot be reached
event though it is a correct command.

Use `buf->len` as the end condition of the while-loop.

Fixes #74730.

Signed-off-by: Lyle Zhu <lyle.zhu@nxp.com>