serial: of_serial: check the return value of clk_prepare_enable()

The function clk_prepare_enable() may fail, and in that case it
does not make sense to proceed.  Let's check its return code and
error out if it is a negative value.

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