iio: dac: ad3530r: Fix AD3531/AD3531R powerdown mode strings

iio: dac: ad3530r: Fix AD3531/AD3531R powerdown mode strings

The AD3531/AD3531R has different output operating modes from the
AD3530/AD3530R. According to the AD3531/AD3531R datasheet, the
powerdown modes are:
  01: 500 Ohm output impedance
  10: 3.85 kOhm output impedance
  11: 16 kOhm output impedance

The driver currently uses the AD3530R modes (1k, 7.7k, 32k) for all
variants, which is incorrect for AD3531/AD3531R.

Add AD3531R-specific powerdown mode strings and assign them to the
AD3531/AD3531R chip variants.

Fixes: 93583174a3df ("iio: dac: ad3530r: Add driver for AD3530R and AD3531R")
Signed-off-by: Kim Seer Paller <kimseer.paller@analog.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>