staging: speakup: fix wraparound in uaccess length check

If softsynthx_read() is called with `count < 3`, `count - 3` wraps, causing
the loop to copy as much data as available to the provided buffer. If
softsynthx_read() is invoked through sys_splice(), this causes an
unbounded kernel write; but even when userspace just reads from it
normally, a small size could cause userspace crashes.

Fixes: 425e586c ("speakup: add unicode variant of /dev/softsynth")
Cc: stable@vger.kernel.org
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>