ptp: fix missing break in switch

It seems that a *break* is missing in order to avoid falling through
to the default case. Otherwise, checking *chan* makes no sense.

Fixes: 72df7a72

 ("ptp: Allow reassigning calibration pin function")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>