iio: adc: ad7768-1: Move buffer allocation to a separate function

iio: adc: ad7768-1: Move buffer allocation to a separate function

This change moves the buffer allocation and related trigger allocation
in a separate function, making space for adding another type of iio
buffer if needed.

Reviewed-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Sergiu Cuciurean <sergiu.cuciurean@analog.com>
Signed-off-by: Jonathan Santos <Jonathan.Santos@analog.com>
Link: https://patch.msgid.link/11c1777b406875ce1a7216dc4b094ff99af8da7f.1744325346.git.Jonathan.Santos@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>