iwlwifi: limit dwell time more strictly

The dwell time for scanning is currently limited
so that it fits into the timings inside the ucode
when that is tracking DTIM/beacon periods for the
AP(s) it's connected to.

However, when it's connected to two APs, those
may be in lockstep, for example if they both have
a DTIM interval of 100 TU, then one could be 50
TU after the other, leaving only 50 TU free to
be used by scanning.

Since we can't know how far apart they are the
only option is to restrict to 1/2 of the minium
of the two APs.

In theory, it would be possible to not use 1/2 of
the minimum but take into account that if they
have different intervals then there will be a bit
more time since they can't be in lockstep, but as
they will have 100 TU intervals in practice that
complex calculation will probably just result in
hard-to-find bugs.

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