iwlwifi: mvm: fix netdetect starting/stopping for unified images

With unified images, we need to make sure the net-detect scan is
stopped after resuming, since we don't restart the FW.  Also, we need
to make sure we check if there are enough scan slots available to run
it, as we do with other scans.

Fixes: commit 23ae6128

 ("iwlwifi: mvm: Do not switch to D3 image on suspend")
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>