firewire: core: use more outbound tlabels

Tlabel is a 6 bits wide datum.  Wrap it after 63 rather than 31 for more
safety against transaction label exhaustion and potential responders'
transaction layer bugs.  (As noted by Guus Sliepen, this change requires
an expansion of tlabel_mask to 64 bits.)

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