greybus: gpio: refuse to set value of input pins

Add warning and refuse to set output value for pin configured as input,
as the result of such an operation is undefined.

Remove incorrect todo-comment suggesting that the driver could
implicitly switch direction as part of the call.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>