iwlwifi: mvm: remove unnecessary check in iwl_mvm_is_d0i3_supported()

The d0i3_mode element is never set to IWL_D0I3_OFF, so it's not
necessary to check it in iwl_mvm_is_d0i3_supported().

Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>