stand: Change disk_fmtdev to take a struct devdesc *

We do a number of games with ploymorphism for different types struct
*devdesc. Adjust one place that this affects to take the address of the
base class (most others have void * at the moment). This is more type
safe than a bare void *.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D35914