ppc405_uc: Fix buffer overflow

Report from smatch:

ppc405_uc.c:209 dcr_read_pob(12) error: buffer overflow 'pob->besr' 2 <= 2
ppc405_uc.c:232 dcr_write_pob(12) error: buffer overflow 'pob->besr' 2 <= 2

The old code reads and writes besr[POB0_BESR1 - POB0_BESR0] or besr[2]
which is one too much.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Alexander Graf <agraf@suse.de>
