Move the tx cpu dma ring index update out of transmit loop of
airoha_dev_xmit routine in order to not start transmitting the packet
before it is fully DMA mapped (e.g. fragmented skbs).

Fixes: 23020f04 ("net: airoha: Introduce ethernet support for EN7581 SoC")
Reported-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20241004-airoha-eth-7581-mapping-fix-v1-1-8e4279ab1812@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>