iwlwifi: pcie: support host commands in new transport

Code is basically the same, with a cleanups of old narrow host
command, ampg workarounds, some cosmetic stuff, and usage of
TFH functions when accessing TFD queues.
This enables also the cleanup of iwl_pcie_tfd_set_tb() since
now it won't be called anywhere in the a000 data path

Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>