iwlwifi: move iwl_txq and substructures to a common trans header

The txq code is not directly related to the PCIe transport, so move the
structures it uses to the common iwl-trans.h header.

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.20200529092401.d9d0082b8369.I8298f6e83804c1ea99217a79d95d23ef68b184d4@changeid