crypto: cavium - cpt_bind_vq_to_grp could return an error code

cpt_bind_vq_to_grp() could return an error code. However, it currently
returns a u8. This produce the static checker warning.

drivers/crypto/cavium/cpt/cptpf_mbox.c:70 cpt_bind_vq_to_grp() warn: signedness bug returning '(-22)'

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: George Cherian <george.cherian@cavium.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>