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

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>

drivers/infiniband/hw/mlx4/qp.c

@@ -1747,7 +1747,7 @@ static int build_mlx_header(struct mlx4_ib_sqp *sqp, struct ib_send_wr *wr,
 	int spc;
 	int i;
 	int err = 0;
-	u16 vlan;
+	u16 vlan = 0xffff;
 	bool is_eth;
 	bool is_vlan = false;
 	bool is_grh;