crypto: octeontx2 - fix signedness bug in cptvf_register_interrupts()

The "num_vec" has to be signed for the error handling to work.

Fixes: 19d8e8c7 ("crypto: octeontx2 - add virtual function driver support")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>