RDMA/mlx5: Fix NULL string error

commit dab994bcc6 upstream.

checkpath is complaining about NULL string, change it to 'Unknown'.

Fixes: 37aa5c36aa ("IB/mlx5: Add UARs write-combining and non-cached mapping")
Signed-off-by: Shay Drory <shayd@nvidia.com>
Link: https://lore.kernel.org/r/8638e5c14fadbde5fa9961874feae917073af920.1695203958.git.leonro@nvidia.com
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/infiniband/hw/mlx5/main.c

@@ -2095,7 +2095,7 @@ static inline char *mmap_cmd2str(enum mlx5_ib_mmap_cmd cmd)
 	case MLX5_IB_MMAP_DEVICE_MEM:
 		return "Device Memory";
 	default:
-		return NULL;
+		return "Unknown";
 	}
 }