dm: spi: Correct SPI claim/release_bus() methods

These methods should be passed a slave device, not a bus. This matches the
old SPI interface. It is important to know which device is claiming the bus
so passing a bus is not that useful.

Reported-by: Haikun Wang <haikun.wang@freescale.com>
Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Peng Fan <Peng.Fan@freescale.com>
Reviewed-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>