USB: mos7840: fix tiocmget error handling

commit a91ccd26e75235d86248d018fe3779732bcafd8d upstream.

Make sure to return errors from tiocmget rather than rely on
uninitialised stack data.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>