iio/adc: ingenic: rename has_aux2 to has_aux_md

iio/adc: ingenic: rename has_aux2 to has_aux_md

The jz4760(b) socs have 3 aux channels.

The purpose of has_aux2 is to set the MD bits used to select
the AUX channel to be sampled, not to describe the hardware.

Rename it to a more appropriate name.

Signed-off-by: Christophe Branchereau <cbranchereau@gmail.com>
Reviewed-by: Paul Cercueil <paul@crapouillou.net>
Link: https://lore.kernel.org/r/20210726082033.351533-2-cbranchereau@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>