iwlwifi: mvm: fix scan offload for BGN SKU

BGN SKU won't scan on 5.2GHz obviously, but the firmware
still expects the driver to reserve space for the the probe
request for the 5.2GHz band.
Fix this by allocating space and leave it empty.

This fixes https://bugzilla.kernel.org/show_bug.cgi?id=69541



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