IB/mlx4: Fix the size of a buffer in add_port_entries()

IB/mlx4: Fix the size of a buffer in add_port_entries()

In order to be sure that 'buff' is never truncated, its size should be
12, not 11.

When building with W=1, this fixes the following warnings:

  drivers/infiniband/hw/mlx4/sysfs.c: In function ‘add_port_entries’:
  drivers/infiniband/hw/mlx4/sysfs.c:268:34: error: ‘sprintf’ may write a terminating nul past the end of the destination [-Werror=format-overflow=]
    268 |                 sprintf(buff, "%d", i);
        |                                  ^
  drivers/infiniband/hw/mlx4/sysfs.c:268:17: note: ‘sprintf’ output between 2 and 12 bytes into a destination of size 11
    268 |                 sprintf(buff, "%d", i);
        |                 ^~~~~~~~~~~~~~~~~~~~~~
  drivers/infiniband/hw/mlx4/sysfs.c:286:34: error: ‘sprintf’ may write a terminating nul past the end of the destination [-Werror=format-overflow=]
    286 |                 sprintf(buff, "%d", i);
        |                                  ^
  drivers/infiniband/hw/mlx4/sysfs.c:286:17: note: ‘sprintf’ output between 2 and 12 bytes into a destination of size 11
    286 |                 sprintf(buff, "%d", i);
        |                 ^~~~~~~~~~~~~~~~~~~~~~

Fixes: c1e7e466120b ("IB/mlx4: Add iov directory in sysfs under the ib device")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/0bb1443eb47308bc9be30232cc23004c4d4cf43e.1695448530.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Leon Romanovsky <leon@kernel.org>