net: bcmgenet: fix off-by-one in bcmgenet_put_txcb

net: bcmgenet: fix off-by-one in bcmgenet_put_txcb

The write_ptr points to the next open tx_cb. We want to return the
tx_cb that gets rewinded, so we must rewind the pointer first then
return the tx_cb that it points to. That way the txcb can be correctly
cleaned up.

Fixes: 876dbadd53a7 ("net: bcmgenet: Fix unmapping of fragments in bcmgenet_xmit()")
Signed-off-by: Justin Chen <justin.chen@broadcom.com>
Reviewed-by: Nicolai Buchwitz <nb@tipi-net.de>
Link: https://patch.msgid.link/20260406175756.134567-2-justin.chen@broadcom.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>