RDMA/ocrdma: Suppress a compiler warning

This patch avoids that the following compiler warning is reported when
building with gcc 8 and W=1:

In function 'ocrdma_mbx_get_ctrl_attribs',
    inlined from 'ocrdma_init_hw' at drivers/infiniband/hw/ocrdma/ocrdma_hw.c:3224:11:
drivers/infiniband/hw/ocrdma/ocrdma_hw.c:1368:3: warning: 'strncpy' output may be truncated copying 31 bytes from a string of length 31 [-Wstringop-truncation]
   strncpy(dev->model_number,
   ^~~~~~~~~~~~~~~~~~~~~~~~~~
    hba_attribs->controller_model_number, 31);
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Acked-by: Selvin Xavier <selvin.xavier@broadcom.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>