sunxi: rsb: Move rsb_set_device_mode() call to rsb_init()

It turns out that the device_mode_data is rsb specific, rather then slave
specific, so integrate the rsb_set_device_mode() call into rsb_init().

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>