iwlwifi: use minimal time for radio reset scan

The effect of using a short single-channel scan
to reset the radio is that scanning a channel
that isn't in use needs to re-tune the radio.
This means that the dwell time is irrelevant,
so use a shorter time.

While at it, clean up the code for this a bit.

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