ptp4l: Set clock frequency on start.

Due to a bug in older kernels, frequency reading may silently fail and
return 0. Set the frequency to the read value to make sure the servo
has the actual frequency of the clock.

Signed-off-by: Miroslav Lichvar <mlichvar@redhat.com>
ptp4l: Read system clock maximum frequency adjustment.

Signed-off-by: Miroslav Lichvar <mlichvar@redhat.com>
Fix compiler warnings with -O2.

Signed-off-by: Miroslav Lichvar <mlichvar@redhat.com>