IB/mlx4: Fix SL to 802.1Q priority-bits mapping for IBoE

For IBoE, SLs 0-7 are mapped to Ethernet 802.1Q user priority bits
(pbits) which are part of the VLAN tag, SLs 8-15 are reserved.

Under Ethernet, the ConnectX firmware treats (decode/encode) the four
bit SL field in various constructs such as QPC / UD WQE / CQE as PPP0
and not as 0PPP. This correlates well to the fact that within the
vlan tag the pbits are located in bits 15-13 and not 12-14.

The current code wasn't consistent around that area - the
encoding was correct for the IBoE QPC.path.schedule_queue field,
but was wrong for IBoE CQEs and when MLX header was built.

These inconsistencies resulted in wrong SL <--> wire 802.1Q pbits
mapping, which is fixed by using SL <--> PPP0 all around the place.

Signed-off-by: Oren Duer <oren@mellanox.co.il>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>