iio: Drop Duplicated "mount-matrix" parameter

All of the users of iio_read_mount_matrix() are using the very same
property name. Moreover, the property name is hard coded in the API
documentation.

Make this clear and avoid duplication now and in the future.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Sean Nyekjaer <sean@geanix.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20210518112546.44592-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>