drivers/spi/spi.c: use DEVICE_ID_DYNAMIC when allocating a struct device_d
This causes allocation of a free id and avoids conflicts if multiple
identical SPI devices are attached.
Signed-off-by: Jan Luebbe <jlu@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>