ptp: Fix clock_getres() implementation

The clock_getres() function must return the resolution in the timespec
argument and return 0 for success.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: John Stultz <john.stultz@linaro.org>
Cc: stable@kernel.org
Cc: Richard Cochran <richard.cochran@omicron.at>