iwlwifi: iwl-trans: move all txcmd init to trans alloc

txcmd fields is not directly related to the PCIe transport,
so move to the common iwl_trans_alloc function.

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.20200924162105.862ef88d1ab2.Iba220a962b5d6d05c030b9275d97a89202d055dc@changeid


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