iwlwifi: mvm: tell firmware about required LTR delay

Some (integrated) devices need a longer LTR delay than the firmware
would typically apply, tell it about that.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20200418110539.24276ae2ad61.I8831a538f75893d5cee47b4a81f4b9b7fd0e8bea@changeid