ravb: skip gPTP start/stop on R-Car gen3

When adding support for the  R-Car gen3 gPTP active in configuration mode,
some call sites of ravb_ptp_{init|stop}() were missed due to an oversight.
Add  checks for the R-Car gen2 SoCs around these...

Fixes: f5d7837f ("ravb: ptp: Add CONFIG mode support")
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>