powerpc/oprofile: fix potential buffer overrun in op_model_cell.c

Fix potential initial_lfsr buffer overrun.
Writing past the end of the buffer could happen when index == ENTRIES

Signed-off-by: Denis Kirjanov <dkirjanov@kernel.org>
Cc: stable@kernel.org
Signed-off-by: Robert Richter <robert.richter@amd.com>