be2net: signedness bug in be_msix_enable()

"num_vec" needs to be signed for the error handling to work.

Fixes: e261768e

 ('be2net: support asymmetric rx/tx queue counts')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Sathya Perla <sathya.perla@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>