serial: of_serial: use devm_clk_get() instead of clk_get()

The probe method of this driver calls clk_get(), but clk_put() is
missing from the remove callback.

Using the managed clk function is easier than fixing other parts.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>