iwlagn: prepare for multi-TB commands

In a subsequent patch, I want to make commands use
multiple TBs in a TFD. This is a simple change to
prepare the data structures for this, with as of
now still just a single TB supported.

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