iwlwifi: mvm: provide the actual number of frames for the SP len

In the end, the firmware doesn't want the SP len as present
in the WMM IE, but rather the actual number of frames.

Fixes: bd3c6cf901a8 ("iwlwifi: mvm: tell the firmware about the U-APSD parameters")
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>