Fix serial console on SNI RM400 machines

sccnxp driver doesn't get the correct uart clock rate, if CONFIG_HAVE_CLOCK
is disabled. Correct usage of clk API to make it work with/without it.

Fixes: 90efa75f (serial: sccnxp: Using CLK API for getting UART clock)

Suggested-by: Russell King - ARM Linux <linux@armlinux.org.uk>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>