net: renesas: rswitch: Fix timestamp feature after all descriptors are used

The timestamp descriptors were intended to act cyclically. Descriptors
from index 0 through gq->ring_size - 1 contain actual information, and
the last index (gq->ring_size) should have LINKFIX to indicate
the first index 0 descriptor. However, the LINKFIX value is missing,
causing the timestamp feature to stop after all descriptors are used.
To resolve this issue, set the LINKFIX to the timestamp descritors.

Reported-by: Phong Hoang <phong.hoang.wz@renesas.com>
Fixes: 33f5d733 ("net: renesas: rswitch: Improve TX timestamp accuracy")
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>