stand: Small comment correction

While in theory, once upon a time, dv_type was arch specific, that's
never been the case in FreeBSD (and certaintly isn't in the surviving
drivers). Remove that notation.

Sponsored by: Netflix
Reviewed by: tsoome
Differential Revision: https://reviews.freebsd.org/D35911

(cherry picked from commit 079f02e8c035f12779a57f10b274253a701267e5)