mips_busdma: fix mips errors after merge

The change "vm_extern: use standard address checkers everywhere"
introduced bugs, as well as features, into head. After it was
introduced, but before those bugs were addressed, mips support was
dropped from head, so those mips bugs were never fixed in head. Now,
that change and those fixes have been merged into stable/13. This
change fixes two of those bugs that were fixed everywhere else but
never in mips.

Reviewed by: alc
Differential Revision: https://reviews.freebsd.org/D35793