iio: adc: at91-sama5d2_adc: merge buffer & trigger init into a function

iio: adc: at91-sama5d2_adc: merge buffer & trigger init into a function

This change is mostly cosmetic, but it's also a pre-cursor to the
the change for 'iio_buffer_set_attrs()', where the helper gets updated to
better support multiple IIO buffers for 1 IIO device.

The only functional change is that the error message for the trigger alloc
failure is bound to the parent device vs the IIO device object.

Also, the new at91_adc_buffer_and_trigger_init() function was moved after
the definition of the 'at91_adc_fifo_attributes'.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20200929125949.69934-3-alexandru.ardelean@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>