iio: adc: aspeed: completes the bitfield declare.

This patch completes the declare of ADC register bitfields and uses the
same prefix ASPEED_ADC_* for these bitfields. In addition, tidy up space
alignment of the codes.

Signed-off-by: Billy Tsai <billy_tsai@aspeedtech.com>
Link: https://lore.kernel.org/r/20210831071458.2334-4-billy_tsai@aspeedtech.com


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