serial: earlycon: Move ->uartclk initialize

Initializing the ->uartclk field is not related to option parsing;
relocate from parse_options() to setup_earlycon() (which mirrors the
behavior of of_setup_earlycon()).

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>