iio: adc: nxp-sar-adc: Fix DMA channel leak in trigger mode

iio: adc: nxp-sar-adc: Fix DMA channel leak in trigger mode

The DMA channel was requested in nxp_sar_adc_buffer_postenable() but
was only released in nxp_sar_adc_buffer_software_do_predisable().
This caused a DMA channel resource leak when operating in trigger mode.

Fix this by moving dma_request_chan() from
nxp_sar_adc_buffer_postenable() into
nxp_sar_adc_buffer_software_do_postenable(), ensuring the DMA channel
is only requested in software mode.

Fixes: 4434072a893e ("iio: adc: Add the NXP SAR ADC support for the s32g2/3 platforms")
Signed-off-by: Felix Gu <ustc.gu@gmail.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>