ptp: Introduce strict checking of external time stamp options.

User space may request time stamps on rising edges, falling edges, or
both.  However, the particular mode may or may not be supported in the
hardware or in the driver.  This patch adds a "strict" flag that tells
drivers to ensure that the requested mode will be honored.

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