iwlwifi: move all bus-independent TX functions to common code

After moving out all Tx fields not related to pcie-bus
it's time to move the code to a common place.

We also rename all pcie functions name to txq.

Signed-off-by: Mordechay Goodstein <mordechay.goodstein@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20200930161256.3947a5276003.I3fe1bec2b25a965a49532df288f47b8b59eb1500@changeid


Signed-off-by: Luca Coelho <luciano.coelho@intel.com>