mips: fcmpset: do not spin on sc failure

For ll/sc architectures, atomic(9) allows failure modes where *old == val
due to write failure and callers should compensate for this. Do not retry on
failure, just leave 0 in ret and fail the operation if we couldn't sc it.
This lets the caller determine if it should retry or not.

Reviewed by: kib
Looks ok: imp
Differential Revision: https://reviews.freebsd.org/D21836