Cr50: Disable spstest command.
Cr50: Disable spstest command.

This command was used for early testing of the SPI slave
interface. It's no longer needed, so disable it to save space.

BUG=none
BRANCH=none
TEST=make buildall; test on Cr50 hardware

Change-Id: If730f909c7361b1179e23b2bf1d07c9d51aa6e8f
Signed-off-by: Bill Richardson <129945214b1d548d8e49b6c29c43094f8c78057f@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/373619
Reviewed-by: Vadim Bendebury <5515d6d2d0829cbe0dd0dcf2094aaded06d58514@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec