stand: Fix logo alpha

The alpha wasn't done properly and some white stripes could be seen.

Differential Revision: https://reviews.freebsd.org/D45933
Reviewed by: imp. tsoome
Sponsored by: Beckhoff Automation GmbH & Co. KG

(cherry picked from commit 887062c1da4a4c04a2b3987ec9a4e2a5902122d5)