staging:iio:adc:ad7280a: Standardize extended ABI naming

The *_balance_switch_en and *_balance_switch_timer attributes had non
standard prefixes. Use the ext_info framework to automatically
create then with in_voltageX-voltageY_ prefix.

Documentation for these two unusual attributes to follow.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Marcelo Schmitt <marcelo.schmitt1@gmail.com>
Link: https://lore.kernel.org/r/20220206190328.333093-8-jic23@kernel.org