wifi: iwlwifi: mvm: correctly access HE/EHT sband capa

wifi: iwlwifi: mvm: correctly access HE/EHT sband capa

We can't just dereference the sband->iftype_data pointer,
that's an array so we need to access the right entry. Use
the previously introduced helper functions to do that.

There are also cases, e.g. when loading with disable_11ax=1,
where the pointer might be NULL but we still attempt to use
it, causing a crash.

Fixes: 529281bdf0fc ("iwlwifi: mvm: limit TLC according to our HE capabilities")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230614123446.a1f2b17ee39b.I8808120be744be8804815ce9e3e24ce6d2b424e3@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>