iio: adc: ad7380: disable offload before using SPI bus

Move disabling of the SPI offload before attempting to use the SPI bus
to write a register in ad7380_offload_buffer_predisable().

This caused a crash in the spi_engine_irq() interrupt handler due to
being in an invalid state.

Fixes: bbeaec81 ("iio: ad7380: add support for SPI offload")
Signed-off-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Reviewed-by: Angelo Dureghello <adureghello@baylibre.com>
Link: https://patch.msgid.link/20250320-iio-adc-ad7380-fix-spi-offload-buffer-predisable-v1-1-6912ac8c0ae0@baylibre.com


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