iwlwifi: mvm: stop claiming NL80211_EXT_FEATURE_SET_SCAN_DWELL

We've removed the code that handles it appropriately, so stop
claiming we support it.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/iwlwifi.20201008180656.3b8936035273.I7799caa225295de44558e9da093b720afe606359@changeid