iwlwifi: mvm: fix a comment about the SP length

The SP length in the ADD_STA command is an actual number of
frames, and not the SP len as it appears in the WME IE.
Fix that comment. The actual code is fine.

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