spi/bitbang: don't error out if there is no setup callback provided

It's perfectly valid not to have a setup callback when the probe routine
does all the needed things. So don't even check for this case and trust
the caller.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Mark Brown <broonie@linaro.org>