iwlwifi: mvm: Fix active-to-powersave transition time units

Active to power save mode transition time for TX/RX in the power
table command is in microseconds, fix the units in the driver.

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