net: ethernet: mtk_eth_soc: fix usage of foe_entry_size

net: ethernet: mtk_eth_soc: fix usage of foe_entry_size

As sizeof(hwe->data) can now longer be used as the actual size depends
on foe_entry_size, in commit e17be858137834
("net: ethernet: mtk_eth_soc: add foe_entry_size to mtk_eth_soc") the
use of sizeof(hwe->data) is hence replaced.

However, replacing it with ppe->eth->soc->foe_entry_size is wrong as
foe_entry_size represents the size of the whole descriptor and not just
the 'data' field.

Fix this by subtracing the size of the only other field in the struct
'ib1', so we actually end up with the correct size to be copied to the
data field.

Reported-by: Chen Minqiang <ptpt52@gmail.com>
Fixes: e17be858137834 ("net: ethernet: mtk_eth_soc: add foe_entry_size to mtk_eth_soc")
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://lore.kernel.org/r/YzBqPIgQR2gLrPoK@makrotopia.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>