ieee1394: add hpsb_node_read() and hpsb_node_lock()
These will be used by the firedtv driver. Like hpsb_node_write() they
are much better APIs for high-level drivers than hpsb_write() and its
siblings --- easier to use correctly and also terser.
Unlike hspb_node_write(), the two new functions will only be used by
one call site. Hence make them static inline instead of exported
symbols.
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>