firewire: cdev: add more flexible cycle timer ioctl

The system time from CLOCK_REALTIME is not monotonic, hence problematic
for the main user of the FW_CDEV_IOC_GET_CYCLE_TIMER ioctl.  This issue
exists in its successor ABI, i.e. raw1394, too.
http://subversion.ffado.org/ticket/242



We now offer an alternative ioctl which lets the caller choose between
CLOCK_REALTIME, CLOCK_MONOTONIC, and CLOCK_MONOTONIC_RAW as source of
the local time, very similar to the clock_gettime libc function.  The
format of the local time return value matches that of clock_gettime
(seconds and nanoseconds, instead of a single microseconds value from
the existing ioctl).

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