spi: xilinx: Use pdev->id instead of pdev->dev.id for the SPI bus number

The Xilinx SPI driver initializes the bus_num field of the SPI master to
pdev->dev.id. pdev->dev.id is always 0 for platform devices which causes bus
number conflicts for the SPI controller when creating multiple device instances
of the driver. Instead use pdev->id which will have the assignt platform device
id. If the device is instantiated via devicetree pdev->id is -1 which will cause
the SPI core to dynamically assign a id (or use the id defined via a alias in
the devicetree).

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@linaro.org>