mlx4: Use snprintf instead of complicated strcpy

This fixes a compilation warning in sysfs.c

drivers/infiniband/hw/mlx4/sysfs.c:360:2: warning: 'strncpy' output may be
truncated copying 8 bytes from a string of length 31
[-Wstringop-truncation]

By eliminating the temporary stack buffer.

Signed-off-by: Qian Cai <cai@gmx.us>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>