iwlwifi: pcie: allow to pretend to have Tx CSUM for debug

Allow to configure the driver to pretend to have TX CSUM
offload support. This will be useful to test the TSO flows
that will come in further patches.
This configuration is disabled by default.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>