ppc405_uc: fix a buffer overflow

Fix a buffer overflow, reported by cppcheck:
[/src/qemu/hw/ppc405_uc.c:72]: (error) Buffer access out-of-bounds: bd.bi_s_version

The use of field bi_s_version seems to be a typo, it should be
bi_r_version.

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>