net: tehuti: don't process data if it has not been copied from userspace

The array data is only populated with valid information from userspace
if cmd != SIOCDEVPRIVATE, other cases the array contains garbage on
the stack. The subsequent switch statement acts on a subcommand in
data[0] which could be any garbage value if cmd is SIOCDEVPRIVATE which
seems incorrect to me.  Instead, just return EOPNOTSUPP for the case
where cmd == SIOCDEVPRIVATE to avoid this issue.

As a side note, I suspect that the original intention of the code
was for this ioctl to work just for cmd == SIOCDEVPRIVATE (and the
current logic is reversed). However, I don't wont to change the current
semantics in case any userspace code relies on this existing behaviour.

Detected by CoverityScan, CID#139647 ("Uninitialized scalar variable")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>