iio:adc:mcp3422 Fix incorrect scales table

This patch fixes uncorrect order of mcp3422_scales table, the values
was erroneously transposed.
It removes also an unused array and a wrong comment.

Signed-off-by: Angelo Compagnucci <angelo.compagnucci@gmail.com>
Cc: Stable@vger.kernel.org
Signed-off-by: Jonathan Cameron <jic23@kernel.org>