staging: speakup: safely close tty

Speakup opens tty using tty_open_by_driver. When closing, it calls
tty_ldisc_release but doesn't close and remove the tty itself. As a
result, that tty cannot be opened from user space. This patch calls
tty_release_struct which ensures that tty is safely removed and freed
up. It also calls tty_ldisc_release, so speakup doesn't need to call it.

Signed-off-by: Okash Khawaja <okash.khawaja@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>