mlx4_vnic: set mod param "lro_num" default value to 0 to disable LRO feature

mlx4_vnic: set mod param "lro_num" default value to 0 to disable LRO feature

Three reasons for us to disable Large Receive Offload in
uek4/vnic driver

1. Some of vnic required LRO related kernel code has been removed
   from upstream by commit# 24245a1b055df246dc94517c1a8b1fdfe7668da0

2. uek4/vnic has better performance on bare metal than uek3/vnic
   (with LRO support) when running iperf/tcp

3. LRO has potential performance impact on TCP in virtualized
   environment

Signed-off-by: Qing Huang <qing.huang@oracle.com>