cryptodev: use NULL throttle timer cb for read direction

cryptodev: use NULL throttle timer cb for read direction

Operations on a cryptodev are considered as *write* only, the callback
of read direction is never invoked. Use NULL instead of an unreachable
path(cryptodev_backend_throttle_timer_cb on read direction).

The dummy read timer(never invoked) is already removed here, it means
that the 'FIXME' tag is no longer needed.

Reviewed-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Hanna Czenczek <hreitz@redhat.com>
Signed-off-by: zhenwei pi <pizhenwei@bytedance.com>
Message-Id: <20230728022006.1098509-6-pizhenwei@bytedance.com>
Signed-off-by: Hanna Czenczek <hreitz@redhat.com>