net: renesas: rswitch: Rename rings in struct rswitch_gwca_queue

To add a new ring which is really related to timestamp (ts_ring)
in the future, rename the following members to improve readability:

    ring --> tx_ring
    ts_ring --> rx_ring

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>