net: rswitch: Drop unused argument/return value

Drop unused argument and return value of rswitch_tx_free() to
simplify the code.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: David S. Miller <davem@davemloft.net>