net: vertexcom: mse102x: Fix RX error handling

In case the CMD_RTS got corrupted by interferences, the MSE102x
doesn't allow a retransmission of the command. Instead the Ethernet
frame must be shifted out of the SPI FIFO. Since the actual length is
unknown, assume the maximum possible value.

Fixes: 2f207cbf ("net: vertexcom: Add MSE102x SPI support")
Signed-off-by: Stefan Wahren <wahrenst@gmx.net>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20250430133043.7722-5-wahrenst@gmx.net


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