iwlwifi: mvm: store latest power command for debugfs read

Instead of re-building the power command upon debugfs read,
store the latest command sent to the firmware.
This reduces the code complexity by reducing the number of
entries in the power code.

Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Reviewed-by: Alexander Bondar <alexander.bondar@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>