spi/bitbang: check for setup_transfer during initialization

setup_transfer is mandatory if spi_bitbang_transfer is used, so
check for it during initialization and not each time during runtime.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>