speakup: fix uninitialized flush_lock

speakup: fix uninitialized flush_lock

The flush_lock is uninitialized, use DEFINE_SPINLOCK
to define and initialize flush_lock.

Fixes: c6e3fd22cd53 ("Staging: add speakup to the staging directory")
Reported-by: Hulk Robot <hulkci@huawei.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Link: https://lore.kernel.org/r/20201117012229.3395186-1-yangyingliang@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>