Documentation/spi/* compile warning fix

Fix this warning:

Documentation/spi/spidev_fdx.c: In function `do_msg':
Documentation/spi/spidev_fdx.c:61: warning: cast from pointer to integer of different size
Documentation/spi/spidev_fdx.c:64: warning: cast from pointer to integer of different size

buf should be unsigned long to match native arch on 32-bit x86 compile.

Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>