iio: magnetometer: rm3100: Stop abusing the ->currentmode

This is an internal variable for the core, here it is set to a "default"
value by the driver in order to later be able to perform checks against
it. None of this is needed because this check actually cares about the
buffers being enabled or not. So it is an unproper side-channel access
to the information "are the buffers enabled?", returned officially by
the iio_buffer_enabled() helper. Use this helper instead.

Cc: Song Qiang <songqiang1304521@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/r/20220207143840.707510-3-miquel.raynal@bootlin.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>