panasonic-laptop: avoid overflow in acpi_pcc_hotkey_add()

num_sifr could go negative since acpi_pcc_get_sqty() returns -EINVAL
on error.  Then it could bypass the sanity check (num_sifr > 255).
The subsequent call to kzalloc() would allocate a small buffer, leading
to a memory corruption.

Signed-off-by: Xi Wang <xi.wang@gmail.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>