thunderx: fix potential sign extension issue

Coverity reported a sign extension issue on rq->caching << 26 in the
Linux ThunderX driver. (It appears Coverity Scan doesn't build arm64
objects in FreeBSD.)

As done in Linux, add a cast.

Fixes: 3c0086b81391 ("Raw import of ThunderX VNIC networking driver...")
Sponsored by: The FreeBSD Foundation