tty: amiserial: fix TIOCSSERIAL permission check

tty: amiserial: fix TIOCSSERIAL permission check

commit 1d31a831cc04f5f942de3e7d91edaa52310d3c99 upstream.

Changing the port closing_wait parameter is a privileged operation.

Add the missing check to TIOCSSERIAL so that -EPERM is returned in case
an unprivileged user tries to change the closing-wait setting.

Cc: stable@vger.kernel.org
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20210407102334.32361-9-johan@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>