ptp: Validate requests to enable time stamping of external signals.

Commit 41560658 ("PTP: introduce new versions of IOCTLs")
introduced a new external time stamp ioctl that validates the flags.
This patch extends the validation to ensure that at least one rising
or falling edge flag is set when enabling external time stamps.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>