After r278004 was committed, Bruce Evans noted that the casts were
actually completely unnecessary, here:

https://lists.freebsd.org/pipermail/svn-src-all/2015-February/098478.html

Remove the casts, and just assign &xxx_io_mc_regs[0][0] directly.

Reviewed by: dumbbell
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D1748