iwlwifi: reduce max remain-on-channel duration

Due to the way the PAN parameters are set up, the
maximum duration isn't 1000 but much lower, set it
to 500 which is safe (somewhere around 550 might
be possible.)

Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>