iwlwifi: warn on SKB free w/o op-mode

If this ever happens, and it looks like some code in PCIe is
a bit broken and might lead to this, we want to know without
crashing, so add a WARN_ON_ONCE().

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20210411124418.a18b7443dc55.Ia29836738acf14a55af5504aba90c6fea9fff785@changeid


Signed-off-by: Luca Coelho <luciano.coelho@intel.com>