ieee1394: remove unused struct member from highlevel API

struct hpsb_highlevel's struct module *owner is neither used by the IEEE
1394 core nor set by any of the in-tree drivers or the two out-of-tree
highlevel drivers I know about (dfg1394, mem1394) --- nor is this member
documented.  An unscheduled removal seems acceptable.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>