net: renesas: rswitch: Fix return value in error path of xmit

Fix return value in the error path of rswitch_start_xmit(). If TX
queues are full, this function should return NETDEV_TX_BUSY.

Fixes: 3590918b

 ("net: ethernet: renesas: Add support for "Ethernet Switch"")
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Link: https://lore.kernel.org/r/20230529073817.1145208-1-yoshihiro.shimoda.uh@renesas.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>