regmap: spi: Don't use spi_write_then_read()

Currently SPI reads are implemented using spi_write_then_read(). This is a
convenience API which as well as constructing a SPI message from parameters
basically the same as for a bytestream read operation also bounces things
into a memory buffer to allow callers to use stack or other non-DMAable
memory. Since regmap should already be ensuring that everything can be
DMAed further up the stack this copy is redundant so switch to using the
underlying spi_sync() API with the buffers provided by the core directly.

Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/base/regmap/regmap-spi.c

diff --git a/drivers/base/regmap/regmap-spi.c b/drivers/base/regmap/regmap-spi.c
index 14b1d88..6a3dbc2 100644
--- a/drivers/base/regmap/regmap-spi.c
+++ b/drivers/base/regmap/regmap-spi.c
@@ -94,8 +94,15 @@ static int regmap_spi_read(void *context,
 {
 	struct device *dev = context;
 	struct spi_device *spi = to_spi_device(dev);
+	struct spi_message m;
+	struct spi_transfer t[2] = { { .tx_buf = reg, .len = reg_size, },
+				     { .rx_buf = val, .len = val_size, }, };
 
-	return spi_write_then_read(spi, reg, reg_size, val, val_size);
+	spi_message_init(&m);
+	spi_message_add_tail(&t[0], &m);
+	spi_message_add_tail(&t[1], &m);
+
+	return spi_sync(spi, &m);
 }
 
 static const struct regmap_bus regmap_spi = {