Add capability flags to describe the presence of Ingress Rate Limit unit
registers and an helper function to clear it.
In the meantime, fix a few harmless issues:
- 6185 and 6095 don't have such registers (reserved)
- the previous code didn't wait for the IRL operation to complete
Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com> Reviewed-by: Andrew Lunn <andrew@lunn.ch> Signed-off-by: David S. Miller <davem@davemloft.net>