staging:iio:hmc5843: Remove ability to change operating mode

only continuous mode is supported for now; the driver could/should
be switched to single conversion mode

operating mode should be determined by the way IIO accesses the device
and not exposed explicitly

Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>