spi: bcm2835: Convert to platform remove callback returning void

The .remove() callback for a platform driver returns an int which makes
many driver authors wrongly assume it's possible to do error handling by
returning an error code. However the value returned is (mostly) ignored
and this typically results in resource leaks. To improve here there is a
quest to make the remove callback return void. In the first step of this
quest all drivers are converted to .remove_new() which already returns
void.

Convert this driver from always returning zero in the remove callback to
the void returning variant.

Now that bcm2835_spi_remove returns no error code any more,
bcm2835_spi_shutdown() does the same thing as bcm2835_spi_remove(). So
drop the shutdown function and use bcm2835_spi_remove() as .shutdown
callback.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20230330211022.2460233-1-u.kleine-koenig@pengutronix.de
Signed-off-by: Mark Brown <broonie@kernel.org>