wifi: iwlwifi: mvm: rs-fw: don't crash on missing channel

wifi: iwlwifi: mvm: rs-fw: don't crash on missing channel

This is more of a workaround, with MLO we can get here with
the deflink not assigned. It's not critical right now that
we have this right, so WARN_ON_ONCE() and don't crash until
we can fix this area of the code.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230329100039.2b0d31bdb60b.I39d23c76eec16ac49f6ae3a6d5f7652041bde855@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>