iio: adc: adi-axi-adc: don't allow concurrent enable/disable calls

Add proper mutex guards as we should not be able to disable
the core in the middle of enabling it.

Note there's no need to rush in backporting this as the only user of the
backend does not do anything crazy..

Fixes: 794ef0e5 ("iio: adc: adi-axi-adc: move to backend framework")
Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20240531-dev-axi-adc-drp-v3-1-e3fa79447c67@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/adi-axi-adc.c

@@ -85,6 +85,7 @@ static int axi_adc_enable(struct iio_backend *back)
 	struct adi_axi_adc_state *st = iio_backend_get_priv(back);
 	int ret;
 
+	guard(mutex)(&st->lock);
 	ret = regmap_set_bits(st->regmap, ADI_AXI_REG_RSTN,
 			      ADI_AXI_REG_RSTN_MMCM_RSTN);
 	if (ret)
@@ -99,6 +100,7 @@ static void axi_adc_disable(struct iio_backend *back)
 {
 	struct adi_axi_adc_state *st = iio_backend_get_priv(back);
 
+	guard(mutex)(&st->lock);
 	regmap_write(st->regmap, ADI_AXI_REG_RSTN, 0);
 }