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>