dm: spi: Do not assume first SPI bus

When CONFIG_OF_PRIOR_STAGE is enabled, this workaround was needed
before device_bind_common assigned request numbers sequentially in the
absence of aliases.

Signed-off-by: Thomas Fitzsimmons <fitzsim@fitzsim.org>
Cc: Bin Meng <bmeng.cn@gmail.com>
Cc: Simon Glass <sjg@chromium.org>