net: vertexcom: mse102x: Fix tx_bytes calculation

The tx_bytes should consider the actual size of the Ethernet frames
without the SPI encapsulation. But we still need to take care of
Ethernet padding.

Fixes: 2f207cbf ("net: vertexcom: Add MSE102x SPI support")
Signed-off-by: Stefan Wahren <wahrenst@gmx.net>
Link: https://patch.msgid.link/20241108114343.6174-3-wahrenst@gmx.net


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