spi: bcm2835: Raise maximum number of slaves to 4

The "RevPi Connect Flat" PLC offered by KUNBUS has 4 slaves attached
to the BCM2835 SPI master.  Raise the maximum number of slaves in the
driver accordingly.
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Reviewed-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Link: https://lore.kernel.org/r/01453fd062de2d49bd74a847e13a0781cbf8143d.1578572268.git.lukas@wunner.deSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-bcm2835.c

@@ -68,7 +68,7 @@
 #define BCM2835_SPI_FIFO_SIZE		64
 #define BCM2835_SPI_FIFO_SIZE_3_4	48
 #define BCM2835_SPI_DMA_MIN_LENGTH	96
-#define BCM2835_SPI_NUM_CS		3   /* raise as necessary */
+#define BCM2835_SPI_NUM_CS		4   /* raise as necessary */
 #define BCM2835_SPI_MODE_BITS	(SPI_CPOL | SPI_CPHA | SPI_CS_HIGH \
 				| SPI_NO_CS | SPI_3WIRE)