serial: make serial_stub_* to static functions

Add missing static to serial_stub_puts().

Unexport serial_stub_{getc,tstc} because they are used locally.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
3 files changed