arm64: set the correct partition type in make-memstick.sh

We create a UFS root filesystem using makefs(8), and later pass it to
mkimg(1) when creating the final image. The correct partition type is
freebsd-ufs; the freebsd parition type is for partitions containing a
BSD disklabel.

Reviewed by: emaste
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D26987