RISC-V: correct enum sbi_ext_rfence_fid

The constants in enum sbi_ext_rfence_fid should match the SBI
specification. See
https://github.com/riscv/riscv-sbi-doc/blob/master/riscv-sbi.adoc#78-function-listing

| Function Name               | FID | EID
| sbi_remote_fence_i          |   0 | 0x52464E43
| sbi_remote_sfence_vma       |   1 | 0x52464E43
| sbi_remote_sfence_vma_asid  |   2 | 0x52464E43
| sbi_remote_hfence_gvma_vmid |   3 | 0x52464E43
| sbi_remote_hfence_gvma      |   4 | 0x52464E43
| sbi_remote_hfence_vvma_asid |   5 | 0x52464E43
| sbi_remote_hfence_vvma      |   6 | 0x52464E43

Fixes: ecbacc2a

 ("RISC-V: Add SBI v0.2 extension definitions")
Reported-by: Sean Anderson <seanga2@gmail.com>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Atish Patra <atish.patra@wdc.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>