cxgb4vf: Initialize mdio_addr before using it

In commit 5ad24def

 ("cxgb4vf: Fix ethtool get_settings for VF driver")
mdio_addr of port_info structure was used unininitialzed. Fixing it.

Signed-off-by: Hariprasad Shenai <hariprasad@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>