tty: tty_io: fix driver refcount imbalance on error path

tty_lookup_driver take a reference to the struct tty_driver, but forget
to release it on the error path, lead to a memory leak.
add a tty_driver_kref_put before error return.

Signed-off-by: Lin Yi <teroincn@163.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>