spi/topcliff: Fix uninitialized variable defect

This patch fixes the following build error introduced by commit
65308c46, "spi/topcliff: cleanup for style and conciseness".

drivers/spi/spi_topcliff_pch.c: In function 'pch_spi_process_messages':
drivers/spi/spi_topcliff_pch.c:752: warning: 'data' is used uninitialized in
+this function

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