spi/xilinx: Refer to platform device as pdev in probe() and remove()

spi/xilinx: Refer to platform device as pdev in probe() and remove()

This is a more traditional name and makes things a bit clearer when
referring to actual struct devices as we do frequently during probe().

Signed-off-by: Mark Brown <broonie@linaro.org>
Acked-by: Michal Simek <michal.simek@xilinx.com>