iwlwifi: add 80211 hdr offset to trace data

Every rx mpdu cmd is built from cmd_hdr | 80211_hdr.  The problem is
that the size of cmd_hdr changes with API changes and we don't know
where the 80211_hdr starts.

By adding the size of cmd_hdr dynamically, we can ensure that we always
know how to parse mpdu frames, without dependending on the API changes.

Signed-off-by: Mordechay Goodstein <mordechay.goodstein@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>