net: renesas: rswitch: Fix GWTSDIE register handling

Since the GWCA has the TX timestamp feature, this driver
should not disable it if one of ports is opened. So, fix it.

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>
Signed-off-by: David S. Miller <davem@davemloft.net>