logging: init backend id regardless of autostart

The `id` is basically a compile-time constant. Setting it every
time the backend is enabled is unnecessary. Instead, set it on
`z_log_init()` regardless of whether or not it requires to be
`autostart`ed.

Fixes an issue where the `filter_get`/`filter_set`
accessed the wrong index and displayed the wrong log level when
user accesses the status of an uninitialized backend via:
`log backend <uninitialized_backend> status`.

Also fixes an issue when user tries to list the backends via:
`log list_backends`, where all uninitialized backends will have
ID = 0.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>
Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>
(cherry picked from commit 8dd9d924)