cr50: control adding USB specific code using proper config
cr50: control adding USB specific code using proper config

CONFIG_USB is a better indicator that USB related code needs to be
included.

BRANCH=none
BUG=none

TEST=none - this patch helped compartmentnalize the code when
     debugging bringup on a new platform.

Change-Id: I12ef77325591853d73e2e09f7c491954e272bde9
Signed-off-by: Vadim Bendebury <5515d6d2d0829cbe0dd0dcf2094aaded06d58514@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/291854
Reviewed-by: Bill Richardson <129945214b1d548d8e49b6c29c43094f8c78057f@chromium.org>

fourier49/BIZ_EC,fourier49/BIZ_EC,fourier49/BIZ_EC,fourier49/BIZ_EC