RISC-V: The version of i-ext should be RISCV_UNKNOWN_VERSION when expanding g-ext.

Fix the wrong version of i-ext when expanding g-ext.  This was changed by
the previous patch accidently.

bfd/
    * elfxx-riscv.c (riscv_parse_std_ext): Fixed the wrong versions of
    i-ext when expanding g-ext.