iio:dac:ad5592r: Fix use of true for IIO_SHARED_BY_TYPE

struct iio_chan_spec_ext_info shared element is of type
enum iio_shared_by, not boolean.   It's like the enum value
will for IIO_SHARED_BY_TYPE == 1 == true, hence no actual
problem has been observed.

  CC [M]  drivers/iio/dac/ad5592r-base.o
  491 |   .shared = true,
      |

Fixes: 56ca9db8 ("iio: dac: Add support for the AD5592R/AD5593R ADCs/DACs")
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Acked-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20200722142515.897378-1-jic23@kernel.org