iwlwifi: mvm: Fix avg-power report

On AX200, the average power was showing positive instead of negative, but
otherwise matched the expected RSSI.  Flip the value to negative before
 giving to mac80211.

Signed-off-by: Ben Greear <greearb@candelatech.com>
Link: https://lore.kernel.org/r/20191126214744.1283-1-greearb@candelatech.com


[removed unnecessary check and some unnecessary parentheses]
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>