tty/serial: mark __init early_smh_setup() static

We get 1 warning when building kernel with W=1:
drivers/tty/serial/earlycon-arm-semihost.c:56:12: warning: no previous prototype for 'early_smh_setup' [-Wmissing-prototypes]

In fact, this function is only used in the file in which it is
declared and don't need a declaration, but can be made static.
so this patch marks this function with 'static'.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>