FW dump may be triggered when running init ucode, for example due to a
sysassert. In this case fw_dump_wk may run after mvm is freed, resulting
in a kernel panic.
Fix it by flushing the work.

Fixes: 01b988a708af ("iwlwifi: mvm: allow to collect debug data when restart is disabled")
Cc: <stable@vger.kernel.org> [3.18+]
Signed-off-by: Andrei Otcheretianski <andrei.otcheretianski@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>