wifi: iwlwifi: mvm: rs-fw: properly access sband->iftype_data

wifi: iwlwifi: mvm: rs-fw: properly access sband->iftype_data

We need to look up the correct version of this struct depending
on the interface type, rather than just checking if the pointer
is non-NULL. Fix that.

Fixes: befebbb30af0 ("iwlwifi: rs: consider LDPC capability in case of HE")
Fixes: b009cf71a982 ("iwlwifi: mvm: only enable HE DCM if we also support TX")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230417113648.1fd54368a02c.Ie4db5e8ae224d9a4a63b528da5d63e1b957b9cef@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>