staging:iio:dac:ad5446: Return cached value for 'raw' attribute

We can not read back the value from the device, but we cache the value anyway so
we might as well return the cached value instead of an error.

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