iwlwifi: pcie: avoid variable shadowing in TFD helpers

The various TFD/TB helpers have two code paths depending on the
type of TFD supported, with variable shadowing due to the new if
branches. Move the fall-through code into else branches to avoid
variable shadowing. While doing so, rename some of the variables
and do some other cleanups (like removing void * casts of void *
pointers.)

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