mlx4_core: Make buffer larger to avoid overflow warning

My static checker complains that the sprintf() here can overflow.

	drivers/infiniband/hw/mlx4/main.c:1836 mlx4_ib_alloc_eqs()
	error: format string overflow. buf_size: 32 length: 69

This seems like a valid complaint.  The "dev->pdev->bus->name" string
can be 48 characters long.  I just made the buffer 80 characters instead
of 69 and I changed the sprintf() to snprintf().

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>