ieee1394: unroll a weird macro

This is a coding style touch-up for ieee1394's handle_incoming_packet().

A preprocessor macro contained hardwired variable names and, even worse,
the 'break' keyword.  This macro is now unrolled and removed.

Also, all 'break's which had the effect of a return are replaced by
return.  And a FIXME comment is brought up to date.

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