iwlwifi: mvm: Fix the keep_alive calculation

The driver must set the keep alive period regardless of power
management state. The keep alive period must be greater or equal to
both the NIC's maximum sleep period, and the listen interval.
However, we were confusing time units (TU) and msec, so fix that.

Signed-off-by: Avri Altman <avri.altman@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>