iwlwifi: decouple PCIe transport from mac80211

The PCIe transport needs to store two pointers in each TX SKB, and
currently assumes mac80211's ieee80211_tx_info is present in the CB
to do that.

In order to remove that assumption, have the opmodes pass in the
offset to where the pointers can be stored in the CB and use the
offset in the PCIe code.

To make the disentanglement complete, remove mac80211.h includes
from everywhere in the generic iwlwifi code. This required adding
an include of cfg80211.h in one place.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>