cimx/sb800: fix pedantic gcc error

A cast did not work for me, but this variable did.
This is one of the many issues with building e3501 I'm
running into.

Change-Id: Ifb19a17770604f2d63dfef762d08200add77ee34
Signed-off-by: Ronald G. Minnich <rminnich@gmail.com>
Reviewed-on: http://review.coreboot.org/7122
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>