wifi: iwlwifi: validate payload length in iwl_pnvm_complete_fn

wifi: iwlwifi: validate payload length in iwl_pnvm_complete_fn

iwl_pnvm_complete_fn() casts pkt->data directly to
struct iwl_pnvm_init_complete_ntfy and reads the status field
without first verifying that the firmware notification payload
is large enough to contain that structure.

Add a WARN_ON_ONCE check against sizeof(*pnvm_ntf) and return
early without reading uninitialised memory if the payload is too
short.

Fixes: b3e4c0f34c17 ("iwlwifi: move PNVM implementation to common code")
Assisted-by: GitHubCopilot:gpt-5.3-codex
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Miri Korenblit <miriam.rachel.korenblit@intel.com>
Link: https://patch.msgid.link/20260715215523.7f2a669e5c75.I00465dcfcbccb250ae9af2d9bb305e24de1ba394@changeid