spi: bitbang: Make chipselect callback optional

The ->chipselect() callback on the bit-banged SPI library
master is optional if using GPIO descriptors: when using
descriptors exclusively without any native chipselects,
the core does not even call out the the native ->set_cs()
and therefore ->chipselect() on a bit-banged SPI master
will not even be called in this case.

Make sure to respect the SPI_MASTER_GPIO_SS as used by
e.g. spi-gpio.c though: this setting will make the core
handle the chip select using GPIO descriptors *AND* call
the local chipselect handler.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20191205091340.59850-1-linus.walleij@linaro.org
Signed-off-by: Mark Brown <broonie@kernel.org>