IB/mlx4: Fix compiler warning about uninitialized 'vlan' variable

IB/mlx4: Fix compiler warning about uninitialized 'vlan' variable

commit 57d88cffc83a2120266bbe9e72cbba5bd5f33675 upstream.

Building qp.o triggers this gcc warning:

    drivers/infiniband/hw/mlx4/qp.c: In function ‘mlx4_ib_post_send’:
    drivers/infiniband/hw/mlx4/qp.c:1862:62: warning: ‘vlan’ may be used uninitialized in this function [-Wmaybe-uninitialized]
    drivers/infiniband/hw/mlx4/qp.c:1752:6: note: ‘vlan’ was declared here

Looking at the code it is clear 'vlan' is only set and used if 'is_eth'
is non-zero. But by initializing 'vlan' to 0xffff, on

    gcc (Ubuntu 4.7.2-22ubuntu1) 4.7.2

on x86-64 at least, we fix the warning, and the compiler was already
setting 'vlan' to 0 in the generated code, so there's no real downside.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
[ Get rid of unnecessary move of 'is_vlan' initialization.  - Roland ]

Signed-off-by: Roland Dreier <roland@purestorage.com>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>