settings: file: do not create file when loading

File backend can be read-only with the use of `settings_file_src()` API. It
makes no sense to create file when `settings_load()` is called and
registered file backend won't be used for saving files (because
`settings_file_dst()` was not used).

Do not create file during `settings_load()` if it does not exist yet. This
just requires to remove FS_O_CREATE flag in `fs_open()` invocation.

Open file with read-only access, which is now possible after removal of
`FS_O_CREATE` flag.

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>