ethtool: linkinfo: remove an unnecessary NULL check

This code generates a Smatch warning:

    net/ethtool/linkinfo.c:143 ethnl_set_linkinfo()
    warn: variable dereferenced before check 'info' (see line 119)

Fortunately, the "info" pointer is never NULL so the check can be
removed.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>