firewire: Use ktime_get_ts()

do_posix_clock_monotonic_gettime() is a leftover from the initial
posix timer implementation which maps to ktime_get_ts()

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Acked-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Link: http://lkml.kernel.org/r/20140611234607.351283464@linutronix.de


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>