ptp: ocp: Add ability to disable input selectors.

This adds support for the "IN: None" selector, which disables
the input on a sma pin.  This should be compatible with old firmware
(the firmware will ignore it if not supported).

Signed-off-by: Jonathan Lemon <jonathan.lemon@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>