iwlwifi: move dump_fh into common code

This means it can be shared for different transport
layers in the future.

Signed-off-by: Inbal Hacohen <Inbal.Hacohen@intel.com>
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>