accessiblity: speakup: Add missing misc_deregister in softsynth_probe

softsynth_probe misses a call misc_deregister() in an error path, this
patch fixes that.

Signed-off-by: Zheng Bin <zhengbin13@huawei.com>
Link: https://lore.kernel.org/r/20220511032937.2736738-1-zhengbin13@huawei.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>