iwlwifi: mvm: reset mvm->scan_type when firmware is started

If we don't reset the scan type when the firmware is
started, we will think the firmware is still configured
after the interface has been brought down. When we will
bring it up again, we will not configure the scan type
in firmware and it will crash with the following assert:

0x0000100A | ADVANCED_SYSASSERT

Fixes: 355346ba ("iwlwifi: mvm: configure scheduled scan according to traffic conditions")
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>