devlink: Clarifies max_macs generic devlink param

The generic param max_macs documentation isn't clear.
Replace it with a more descriptive documentation

Signed-off-by: Shay Drory <shayd@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>