iwlwifi: pcie: use the cfg we passed to iwl_trans_pcie_alloc()

Instead of using iwl_trans->cfg in iwl_trans_pcie_alloc(), use the
local argument that we received.  This will allow us to not to set the
cfg during iwl_trans_alloc() so it can be decided later.

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