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>

drivers/spi/spi-bitbang.c

@@ -446,8 +446,7 @@ int spi_bitbang_start(struct spi_bitbang *bitbang)
 			master->setup = spi_bitbang_setup;
 			master->cleanup = spi_bitbang_cleanup;
 		}
-	} else if (!master->setup)
-		return -EINVAL;
+	}
 
 	/* driver may get busy before register() returns, especially
 	 * if someone registered boardinfo for devices