iwlwifi: mvm: allow to collect debug data from non-sleepable context

iwl_mvm_fw_dbg_collect allows to collect debug data from
the firmware. Most of the firmware interaction is done in
non-sleepable context. It makes little sense to force the
caller of iwl_mvm_fw_dbg_collect to sleep.
Defer the actual collection to a worker so that this
function will be able to be called from any context.

Reviewed-by: Eran Harary <eran.harary@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>