enic: bug fix: make the set/get netlink VF_PORT support symmetrical

To make get/set netlink VF_PORT truly symmetrical, we need to keep track
of what items are set and only return those items on get.  Previously, the
driver wasn't differentiating between a set of attr with a NULL string,
for example, and not setting the attr at all.  We only want to return
the NULL string if the attr was actually set with a NULL string.  Otherwise,
don't return the attr.

Signed-off-by: Scott Feldman <scofeldm@cisco.com>
Signed-off-by: David S. Miller <davem@davemloft.net>