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.de


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