spi: bcm2835: fix typo in comment

GPIOS_OUT_LOW should be GPIOD_OUT_LOW.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Link: https://lore.kernel.org/r/20191105214134.25142-1-chris.packham@alliedtelesis.co.nz


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