spi/xilinx: Eliminate pdata references from common code.

The current code has the OF binding modifying the platform_data
pointer which it must not do, and the common code doesn't really need
to use a pdata pointer.  This patch eliminates the platform_data
references from the common part of the driver in preparation for
merging the OF and non-OF versions.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Tested-by: Michal Simek <monstr@monstr.eu>