serial: mxs-auart: put the device in mxs_auart_probe()

We call the get_device() in the mxs_auart_probe().
For the balance of the reference count, we should put the
device in the mxs_auart_remove().

Signed-off-by: Huang Shijie <shijie8@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>