staging:iio:dac:ad5446: Remove duplicated write sample functions

AD5620_LOAD and AD5446_LOAD are both 0, so all these three functions are
identical and we can replace them with only one.

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>