bnx2x: Don't log mc removal needlessly

When mc configuration changes bnx2x_config_mcast() can return 0 for
success, negative for failure and positive for benign reason preventing
its immediate work, e.g., when the command awaits the completion of
a previously sent command.

When removing all configured macs on a 578xx adapter, if a positive
value would be returned driver would errneously log it as an error.

Fixes: c7b7b483 ("bnx2x: Don't flush multicast MACs")
Signed-off-by: Yuval Mintz <Yuval.Mintz@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>