lib/display_options: Fix print_freq

Build without CONFIG_SPL_SERIAL_SUPPORT does not print the cpu freq.
I have seen this in the odroid U3 board, where on boot one sees this:
CPU:   Exynos4412 @  GHz
instead of:
CPU:   Exynos4412 @ 1 GHz

I am assuming that this change was done to get rid of compiler
warnings related to unused variables when building with
CONFIG_SPL_SERIAL_SUPPORT not being defined in an SPL build.

Signed-off-by: Suriyan Ramasami <suriyan.r@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>
Acked-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Joe Hershberger <joe.hershberger@ni.com>