iio: adc: ad4695: Fix call ordering in offload buffer postenable

iio: adc: ad4695: Fix call ordering in offload buffer postenable

ad4695_enter_advanced_sequencer_mode() was called after
spi_offload_trigger_enable(). That is wrong because
ad4695_enter_advanced_sequencer_mode() issues regular SPI transfers to
put the ADC into advanced sequencer mode, and not all SPI offload capable
controllers support regular SPI transfers while offloading is enabled.

Fix this by calling ad4695_enter_advanced_sequencer_mode() before
spi_offload_trigger_enable(), so the ADC is fully configured before the
first CNV pulse can occur. This is consistent with the same constraint
that already applies to the BUSY_GP_EN write above it.

Update the error unwind labels accordingly: add err_exit_conversion_mode
so that a failure of spi_offload_trigger_enable() correctly exits
conversion mode before clearing BUSY_GP_EN.

Fixes: f09f140e3ea8 ("iio: adc: ad4695: Add support for SPI offload")
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Radu Sabau <radu.sabau@analog.com>
Cc: Stable@vger.kernel.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>