Fix USB3.0 decoding windows on Armada38x

Set correct offset for MBUS windows configuration in
USB3.0 interface.

Submitted by: Marcin Wojtas <mw@semihalf.com>
Obtained from: Semihalf
Sponsored by: Stormshield
Reviewed by: loos
Differential revision: https://reviews.freebsd.org/D10721