staging: iio: hmc5843: Set iio name dynamically

Load the driver using the below command:
echo hmc5983 0x1e > /sys/bus/i2c/devices/i2c-?/new_device

In sysfs, the iio name is hmc5843, however the i2c name is hmc5983,
they are inconsistent.

With this patch, the iio name will be the same as the i2c device name

Signed-off-by: Yong Li <sdliyong@gmail.com>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>