spi: spi-fsl-dspi: Add comments around dspi_pop_tx and dspi_push_rx functions

Their names are confusing, since dspi_pop_tx prepares a word to be
written to the PUSHR register, and dspi_push_rx gets a word from the
POPR register.
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20200304220044.11193-5-olteanv@gmail.comSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-fsl-dspi.c

@@ -245,6 +245,10 @@ struct fsl_dspi {
 	struct fsl_dspi_dma			*dma;
 };
 
+/*
+ * Pop one word from the TX buffer for pushing into the
+ * PUSHR register (TX FIFO)
+ */
 static u32 dspi_pop_tx(struct fsl_dspi *dspi)
 {
 	u32 txdata = 0;
@@ -257,6 +261,7 @@ static u32 dspi_pop_tx(struct fsl_dspi *dspi)
 	return txdata;
 }
 
+/* Prepare one TX FIFO entry (txdata plus cmd) */
 static u32 dspi_pop_tx_pushr(struct fsl_dspi *dspi)
 {
 	u16 cmd = dspi->tx_cmd, data = dspi_pop_tx(dspi);
@@ -269,6 +274,7 @@ static u32 dspi_pop_tx_pushr(struct fsl_dspi *dspi)
 	return cmd << 16 | data;
 }
 
+/* Push one word to the RX buffer from the POPR register (RX FIFO) */
 static void dspi_push_rx(struct fsl_dspi *dspi, u32 rxdata)
 {
 	if (!dspi->rx)