iwlwifi: mvm: don't allow sched scans without matches to be started

The firmware can perform a scheduled scan with not matchsets passed,
but it can't send notification that results were found.  Since the
userspace then cannot know when we got new results and the firmware
wouldn't trigger a wake in case we are sleeping, it's better not to
allow scans without matchsets.

This fixes https://bugzilla.kernel.org/show_bug.cgi?id=110831

Cc: <stable@vger.kernel.org> [3.17+]
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>