arm64/disassem.c: Fix typo sxts to sxts and amount for TYPE_02

The current implementation is wrong, since it unconditionally sets the
amount equal to the <size> field of the instruction. However, when the
<S> bit (scale) is not set, it must be zero.

Also fix a typo, sxts to sxtx, according to the Arm64 documentation.

Reviewed by: mhorne
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D39334