iwlwifi: mvm: set SKIP_OVER_DTIM flag only if needed

setting SKIP_OVER_DTIM when skip_dtim_periods is 0
causes a ucode assert. set the flag only if needed.

Signed-off-by: Eliad Peller <eliadx.peller@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>