iwlwifi: move packet to transport

The base packet structure will (hopefully) be
the same for all transports, but what is in it
differs. Remove the union of all the possible
contents and move the packet itself into the
transport header file. This requires changing
all users of the union to just use pkt->data.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>