iwlwifi: mvm: Rename some power management definitions and methods

Replace SLEEP_OVER_DTIM by SKIP_OVER_DTIM.
Add iwl_mvm prefix to a function name.

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>