iwlwifi: assign directly to iwl_trans->cfg in QuZ detection

We were erroneously assigning the new configuration to a local
variable cfg, but that was not being assigned to anything, so the
change was getting lost.  Assign directly to iwl_trans->cfg instead.

Fixes: 5a8c31aa ("iwlwifi: pcie: fix recognition of QuZ devices")
Cc: stable@vger.kernel.org # 5.2
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>