ar71xx: fix flash fast read on devices using multiple flash chips

Add a callback to allow the SPI driver to indicate which devices fast
flash read works on.

Signed-off-by: Felix Fietkau <nbd@nbd.name>

target/linux/ar71xx/patches-4.9/461-spi-ath79-add-fast-flash-read.patch

@@ -10,10 +10,18 @@
  }
  
  static void ath79_spi_disable(struct ath79_spi *sp)
-@@ -205,6 +202,33 @@ static u32 ath79_spi_txrx_mode0(struct s
+@@ -205,6 +202,38 @@ static u32 ath79_spi_txrx_mode0(struct s
  	return ath79_spi_rr(sp, AR71XX_SPI_REG_RDS);
  }
  
++static bool ath79_spi_flash_read_supported(struct spi_device *spi)
++{
++	if (spi->chip_select || gpio_is_valid(spi->cs_gpio))
++		return false;
++
++	return true;
++}
++
 +static int ath79_spi_read_flash_data(struct spi_device *spi,
 +				     struct spi_flash_read_message *msg)
 +{
@@ -22,9 +30,6 @@
 +	if (msg->addr_width > 3)
 +		return -EOPNOTSUPP;
 +
-+	if (spi->chip_select || gpio_is_valid(spi->cs_gpio))
-+		return -EOPNOTSUPP;
-+
 +	/* disable GPIO mode */
 +	ath79_spi_wr(sp, AR71XX_SPI_REG_FS, 0);
 +
@@ -44,11 +49,12 @@
  static int ath79_spi_probe(struct platform_device *pdev)
  {
  	struct spi_master *master;
-@@ -234,6 +258,7 @@ static int ath79_spi_probe(struct platfo
+@@ -234,6 +263,8 @@ static int ath79_spi_probe(struct platfo
  		master->num_chipselect = pdata->num_chipselect;
  		master->cs_gpios = pdata->cs_gpios;
  	}
 +	master->spi_flash_read = ath79_spi_read_flash_data;
++	master->flash_read_supported = ath79_spi_flash_read_supported;
  
  	sp->bitbang.master = master;
  	sp->bitbang.chipselect = ath79_spi_chipselect;