iwlwifi: mvm: Change number of DTIMs to skip semantics

If skip over DTIMs is enabled the driver can specify number of DTIMs
to skip. This parameter in host-device API implies number of DTIM
periods to skip. For example, to skip one DTIM means sleep over two
DTIM periods. Change semantics accordingly. Change this parameter's
default value.

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>