net/mlx5e: Fix a -Wmaybe-uninitialized warning

As found by Olof's build bot, we gain a harmless warning about a
potential uninitialized variable reference in mlx5:

drivers/net/ethernet/mellanox/mlx5/core/en_tc.c: In function 'parse_tc_fdb_actions':
drivers/net/ethernet/mellanox/mlx5/core/en_tc.c:769:13: warning: 'out_dev' may be used uninitialized in this function [-Wmaybe-uninitialized]
drivers/net/ethernet/mellanox/mlx5/core/en_tc.c:811:21: note: 'out_dev' was declared here

This was introduced through the addition of an 'IS_ERR/PTR_ERR' pair
that gcc is unfortunately unable to completely figure out.

The problem being gcc cannot tell that if(IS_ERR()) in
mlx5e_route_lookup_ipv4() is equivalent to checking if(err) later,
so it assumes that 'out_dev' is used after the 'return PTR_ERR(rt)'.

The PTR_ERR_OR_ZERO() case by comparison is fairly easy to detect
by gcc, so it can't get that wrong, so it no longer warns.

Hadar Hen Zion already attempted to fix the warning earlier by adding fake
initializations, but that ended up not fully addressing all warnings, so
I'm reverting it now that it is no longer needed.

Link: http://arm-soc.lixom.net/buildlogs/mainline/v4.10-rc3-98-gcff3b2c/
Fixes: a42485eb ("net/mlx5e: TC ipv4 tunnel encap offload error flow fixes")
Fixes: a757d108

 ("net/mlx5e: Fix kbuild warnings for uninitialized parameters")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>