firewire: Change struct fw_cdev_iso_packet to not use bitfields.

The struct is part of the userspace interface and can not use
bitfields.  This patch replaces the bitfields with a __u32 'control'
word and provides access macros to set the bits.

Signed-off-by: Kristian Høgsberg <krh@redhat.com>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>