staging:iio:adc:spear rename iodev -> indio_dev to fit with current convention

How we ended up with indio_dev is a mystery, but we have so in the interests
of consistency, move this driver over to that convention.

Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Acked-by: Hartmut Knaack <knaack.h@gmx.de>