Compilation fix for ixgbe_main.c.

Under CONFIG_DCA the compilation is broken since the commit
bd0362dd (ixgbe: Add optional
DCA infrastructure).

IXGBE_SUCCESS is not defined anywhere, replace it with 0.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>