firewire: Include iso timestamp in headers when header_size > 4

Previously, when an iso context had header_size > 4, the iso header
(len/tag/channel/tcode/sy) was passed to userspace followed by quadlets
stripped from the payload.  This patch changes the behavior:
header_size = 8 now passes the header quadlet followed by the timestamp
quadlet.  When header_size > 8, quadlets are stripped from the payload.
The header_size = 4 case remains identical.

Since this alters the semantics of the API, the firewire API version
needs to be bumped concurrently with this change.

This change also refactors the header copying code slightly to be much
easier to read.

Signed-off-by: David Moore <dcm@acm.org>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>