Fix and re-enable active scans on iwm(4) and iwx(4).

Fix and re-enable active scans on iwm(4) and iwx(4).

Ensure that we supply the access point's DTIM period to firmware after
an active scan, as soon as the next beacon arrives. This prevents the
problems which prompted us to keep active scans disabled in our drivers.

Problem debugged and patch by zxystd from the OpenIntelWireless project.
I made some tweaks regarding TIM parsing, which were reviewed by zxystd.

Johannes Berg from Intel has confirmed to me via IRC that firmware
will misbehave if running with a zero DTIM period.

Tested:
8265: jca, stsp
9260: kettenis (possible fallout observed here; will keep an eye on it)
9650: stsp
ax200: zxystd, kevlo, stsp
ax201: stsp

ok kevlo@ kettenis@