bnx2x: downgrade Max BW error message to debug

There are valid configurations where Max BW is configured to zero for
some VNs.
Print the message only if debugging is enabled and do not call the
configuration "illegal".

[v2: use DP(), not BNX2X_DBG_ERR(); recommended by Eilon Greenstein.]

Signed-off-by: Michal Schmidt <mschmidt@redhat.com>
Acked-by: Eilon Greenstein <eilong@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>