serial: mxs-auart: Fix missing clk_disable_unprepare() on error in mxs_get_clks()

Commit 5d7519df ("serial: mxs-auart: Disable clock on error path")
try to disable clock on error path, but still missing the clk_set_rate()
error handling path.

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>