kconfig: refactor conf_set_all_new_symbols() to reduce indentation level

The outer switch statement can be avoided by continue'ing earlier the
loop when the symbol type is neither S_BOOLEAN nor S_TRISTATE.

Remove it to reduce the indentation level by one. In addition, avoid
the repetition of sym->def[S_DEF_USER].tri.

No functional change intended.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>