ptp: Allow reassigning calibration pin function

The ptp pin function programming does not allow calibration pin to change
function. This is problematic on hardware that uses the default calibration
pin for other purposes.

Removing this limitation does not impact calibration if userspace does not
reprogram the calibration pin.

Signed-off-by: Stefan Sørensen <stefan.sorensen@spectralink.com>
Signed-off-by: David S. Miller <davem@davemloft.net>