crypto: mbedtls_shim: Fix concurrency and deadlock issues

- **Fix session mutex handling:** Ensure the `mtls_sessions_lock` is
    always released in `mtls_get_unused_session_index` on failure
    to prevent deadlocks.
- **Protect `in_use` flag:** Added mutex protection when setting
    `in_use = false` in free functions.
- **Cleanup on setup failure:** Added calls to `mbedtls_*_free()`
    in `mtls_session_setup()` when key initialization fails.
- **Free logic fix:** Corrected `if/else` structure in
    `mtls_session_free()` to ensure the correct context is freed.

Signed-off-by: Badr Bacem KAABIA <badrbacemkaabia@gmail.com>