iio: adis_buffer: update device page after changing it

It makes more sense to update the device page as soon as we we
successfully changed it. Moreover, a follow up patch will handle
'spi_sync' error path which would leave 'current_page' in a inconsistent
state.

Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Reviewed-by: Alexandru Ardelean <ardeleanalex@gmail.com>
Link: https://lore.kernel.org/r/20210427085454.30616-3-nuno.sa@analog.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>