spi: bitbang: Make setup_transfer() callback optional

[ Upstream commit 7d0ec8b6f40b356f780b79de63eeafd6b907d68c ]

Some controller drivers have no need of this callback (spi-altera even
causes a NULL pointer dereference because it doesn't register the callback,
falsely assuming that it is already optional).

Fixes: 30af9b55 ("spi/bitbang: Drop empty setup() functions")
Signed-off-by: Pelle Nilsson <per.nilsson@xelmo.com>
Reviewed-by: Ezequiel Garcia <ezequiel.garcia@vanguardiasur.com.ar>
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>