kconfig: move prompt handling to menu_add_prompt() from menu_add_prop()

menu_add_prompt() is the only function that calls menu_add_prop() with
non-NULL prompt.

So, the code inside the if-conditional block of menu_add_prop() can be
moved to menu_add_prompt().

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>