staging:iio:adc:ad7476 unwind use of IIO_CHAN macro.

This macro is being removed to simplify ongoing maintenance
so we need to unwind and remaining users.

V2 has the cleanup Lars-Peter suggested.

Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>