iwlwifi: mvm: handle platform PCIe power limitation

The tx backoff settings used by the thermal throttling mechanism can
also be used for enforcing a limit on the power consumption of the module.

Handle the platform PCIe power limitation by translating the limit
(measured in mw) to its respective tx backoff value. The translation is
module specific.

The resulting tx backoff value is sent to the ucode, and also serves as the
minimal backoff value that can be set by the thermal throttling mechanism.

Signed-off-by: Ido Yariv <idox.yariv@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>