firewire: ohci: factor out iso completion flushing code

In preparation for the following patches that add more flushing, move
the code for flushing accumulated header data into a common function.
The timestamp of the last completed packed is passed through the context
structure instead of a function parameter to allow accessing this value
later outside of the handle_i?_packet functions.

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