iio:magn:hmc5843: Fix passing true where iio_shared_by enum required.

So it's obvious that the code is wrong in passing true, but I'm assuming
that will actually evaluate to 1 and hence IIO_SHARED_BY_TYPE.
The documentation however has this attribute as IIO_SHARED_BY_ALL.
My current assumption is the documentation is wrong.
If anyone knows otherwise please shout out!

Fixes: 7247645f ("iio: hmc5843: Move hmc5843 out of staging")
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Matt Ranostay <matt.ranostay@konsulko.com>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Tested-by: H. Nikolaus Schaller <hns@goldelico.com>
Link: https://lore.kernel.org/r/20200913112546.715624-1-jic23@kernel.org