iio: dac: mcp4725: set iio name property in sysfs

Without this change, the name entity for mcp4725 is missing in
/sys/bus/iio/devices/iio\:device*/name

With this change, name is reported correctly

Signed-off-by: Yong Li <sdliyong@gmail.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>