rapidio: remove choice for enumeration

This is the last use of the tristate choice.

This choice was introduced a decade ago by commit a11650e1


("rapidio: make enumeration/discovery configurable"). Since then,
RAPIDIO_ENUM_BASIC has always been the sole member.

There was no need to have this choice block.

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