iio: adc: ad7292: remove unneeded spi_set_drvdata()

This seems to have been copied from a driver that calls spi_set_drvdata()
but doesn't call spi_get_drvdata().
Setting a private object on the SPI device's object isn't necessary if it
won't be accessed.
This change removes the spi_set_drvdata() call.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Tested-by: Marcelo Schmitt <marcelo.schmitt1@gmail.com>
Reviewed-by: Marcelo Schmitt <marcelo.schmitt1@gmail.com>
Link: https://lore.kernel.org/r/20201119142720.86326-1-alexandru.ardelean@analog.com


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