serial: lantiq: Replace clk_enable/clk_disable with clk generic API

The clk driver has introduced new clock APIs that replace
the existing clk_enable and clk_disable.
- clk_enable() APIs is replaced with clk_prepare_enable()
- clk_disable() API is replaced with clk_disable_unprepare()

Signed-off-by: Songjun Wu <songjun.wu@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>