bootm: fix conditional controlling call to fixup_silent_linux

This function is only defined if CONFIG_SILENT_CONSOLE is set and
CONFIG_SILENT_U_BOOT_ONLY is not set, the call to it should be based
on the same conditions.

Signed-off-by: Paul B. Henson <henson@acm.org>
Acked-by: Simon Glass <sjg@chromium.org>