spi/xilinx_spi: fix incorrect casting

This patch fixes the error exposed by the following build warning:

drivers/spi/xilinx_spi.c: In function 'xilinx_spi_init':
drivers/spi/xilinx_spi.c:411: warning: cast from pointer to integer
of different size

Fixed by change %x to %p in the format string.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>