soc/cavium: Fix overflow before widen

Fix Coverity CID1393974

Change-Id: I39caea8a248d2f1debfca307f6fb7a2fe3e431b1
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/27450
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin Roth <martinroth@google.com>
1 file changed