shell: do not alias atomic_t

Previously, a `uint32_t` was aliased as an `atomic_t`. However,
with #39531, `atomic_t` is now a `long` under the hood, which
is 64-bit on 64-bit platforms.

Fixes #40369

Signed-off-by: Christopher Friedt <[email protected]>

Author: cfriedt

include/shell/shell.h

@@ -659,7 +659,7 @@ BUILD_ASSERT((sizeof(struct shell_backend_ctx_flags) == sizeof(uint32_t)),
  * @internal @brief Union for internal shell usage.
  */
 union shell_backend_cfg {
-	uint32_t value;
+	atomic_t value;
 	struct shell_backend_config_flags flags;
 };