target/ppc: fix Hash64 MMU update of PTE bit R

When updating the R bit of a PTE, the Hash64 MMU was using a wrong byte
offset, causing the first byte of the adjacent PTE to be corrupted.
This caused a panic when booting FreeBSD, using the Hash MMU.

Fixes: a2dd4e83 ("ppc/hash64: Rework R and C bit updates")
Signed-off-by: Leandro Lupori <leandro.lupori@eldorado.org.br>
Signed-off-by: Cédric Le Goater <clg@kaod.org>