iio: adc: ad7380: fix SPI offload trigger rate

Add a special case to double the SPI offload trigger rate when all
channels of a single-ended chip are enabled in a buffered read.

The single-ended chips in the AD738x family can only do simultaneous
sampling of half their channels and have a multiplexer to allow reading
the other half. To comply with the IIO definition of sampling_frequency,
we need to trigger twice as often when the sequencer is enabled to so
that both banks can be read in a single sample period.

Fixes: bbeaec81a03e ("iio: ad7380: add support for SPI offload")
Signed-off-by: David Lechner <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/adc/ad7380.c

···
 	if (ret)
 		return ret;
 
+	/*
+	 * When the sequencer is required to read all channels, we need to
+	 * trigger twice per sample period in order to read one complete set
+	 * of samples.
+	 */
+	if (st->seq)
+		config.periodic.frequency_hz *= 2;
+
 	ret = spi_offload_trigger_enable(st->offload, st->offload_trigger, &config);
 	if (ret)
 		spi_unoptimize_message(&st->offload_msg);