crypto: octeontx2 - prevent underflow in get_cores_bmap()

If we're going to cap "eng_grp->g->engs_num" upper bounds then we should
cap the lower bounds as well.

Fixes: 43ac0b82

 ("crypto: octeontx2 - load microcode and create engine groups")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>