greybus: gpio: handle api changes for 4.5 kernel release

In kernel version 4.5, struct gpio_chip renamed the field 'dev' to
'parent' so handle this properly.

Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Reviewed-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>