iwlwifi: mvm: fix setting HE ppe FW config

The FW expects to get the ppe value for each NSS-BW pair in the same
format as in the he phy capabilities IE, which means that a value of 0
implies ppe should be used for BPSK (mcs 0). If there are no PPE
thresholds in the IE, or if for some NSS-RU pair there's no threshold
set for it (this could happen because it's a variable-sized field), it
means no PPE should not be used for that pair, so the value sent to FW
should be 7 which corresponds to "none".

Fixes: 514c3069 ("iwlwifi: add support for IEEE802.11ax")
Signed-off-by: Naftali Goldstein <naftali.goldstein@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>