iwlwifi: mvm: support description for user triggered fw dbg collection

Add to the user triggered fw debug collection support for describing
the reason of the trigger.
This could be useful for identifying a dump by a unique id, passed as
a description.

Signed-off-by: Golan Ben-Ami <golan.ben.ami@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>