kconfig: Invalidate all symbols after changing to y or m.

Since commit 89b90609 ("kconfig: Add yes2modconfig and
mod2yesconfig targets.") forgot to clear SYMBOL_VALID bit after
changing to y or m, these targets did not save the changes.
Call sym_clear_all_valid() so that all symbols are revalidated.

Fixes: 89b90609 ("kconfig: Add yes2modconfig and mod2yesconfig targets.")
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>