kconfig: use menu_list_for_each_sym() in sym_choice_default()

Choices and their members are associated via the P_CHOICE property.

Currently, sym_get_choice_prop() and expr_list_for_each_sym() are
used to iterate on choice members.

Replace them with menu_for_each_sub_entry(), which achieves the same
without relying on P_CHOICE.

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