platform/mellanox: mlxbf-tmfifo: fix vring_desc.len assignment

platform/mellanox: mlxbf-tmfifo: fix vring_desc.len assignment

Fix warnings reported by sparse, related to incorrect type:
drivers/platform/mellanox/mlxbf-tmfifo.c:284:38: warning: incorrect type in assignment (different base types)
drivers/platform/mellanox/mlxbf-tmfifo.c:284:38:    expected restricted __virtio32 [usertype] len
drivers/platform/mellanox/mlxbf-tmfifo.c:284:38:    got unsigned long

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202404040339.S7CUIgf3-lkp@intel.com/
Fixes: 78034cbece79 ("platform/mellanox: mlxbf-tmfifo: Drop the Rx packet if no more descriptors")
Signed-off-by: David Thompson <davthompson@nvidia.com>
Link: https://lore.kernel.org/r/20250613214608.2250130-1-davthompson@nvidia.com
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>