iwlwifi: remove unnecessary argument to iwl_drv_start()

When iwl_drv_start() is called, trans->cfg must already be set, so
there's no need to pass cfg separately, since it can be accessed
directly from trans->cfg.

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