ionic: fix memory leak of object 'lid'

Currently when netdev fails to allocate the error return path
fails to free the allocated object 'lid'.  Fix this by setting
err to the return error code and jumping to a new label that
performs the kfree of lid before returning.

Addresses-Coverity: ("Resource leak")
Fixes: 4b03b273

 ("ionic: get MTU from lif identity")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Shannon Nelson <snelson@pensando.io>
Signed-off-by: David S. Miller <davem@davemloft.net>