greybus: gpio: add error messages to callbacks not propagating errors

Add error messages on failures to deactivate, set and get operation
handlers as any errors would not be detected by the upper layers (either
because the callbacks are declared void or expected to return a boolean
value).

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