net/mlx5: Expose vhca_id to debugfs

hca_id is an identifier of an mlx5_core instance within the hardware.
This identifier may be required for troubleshooting.

Expose it to debugfs.

Example:

$ cat /sys/kernel/debug/mlx5/mlx5_core.sf.2/vhca_id
0x12

Signed-off-by: Eli Cohen <elic@nvidia.com>
Reviewed-by: Roi Dayan <roid@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>