firewire: cdev: increment ABI version number

so that clients can detect whether the FW_CDEV_IOC_GET_CYCLE_TIMER ioctl
is reliable (on all tested controllers, especially the widely used VIA
controllers, also NEC controllers, see commits b677532b and 1c1517ef).

Also add a comment on the 2.6.32 iso xmit enhancement and on dual-buffer
IR having been disabled in 2.6.33.

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