iwlwifi: mvm: move skb padding reservation earlier

Future changes will require moving the HE radiotap data into
the SKB head, but this means we need to have the alignment
reservation done before that. To prepare, move the alignment
reservation earlier here.

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