iio: adc: stm32-dfsdm: Avoid dereferencing ->currentmode

This is an internal variable of the core, let's use the
iio_buffer_enabled() helper which is exported for the following purpose:
telling if the current mode is a buffered mode, which is precisely what
this driver looks for.

Cc: Olivier Moysan <olivier.moysan@foss.st.com>
Cc: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Link: https://lore.kernel.org/r/20220207143840.707510-4-miquel.raynal@bootlin.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>