net: vertexcom: mse102x: Implement flag for valid CMD

After removal of the invalid command counter only a relevant debug
message is left, which can be cumbersome. So add a new flag to debugfs,
which indicates whether the driver has ever received a valid CMD.
This helps to differentiate between general and temporary receive
issues.

Signed-off-by: Stefan Wahren <wahrenst@gmx.net>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20250509120435.43646-5-wahrenst@gmx.net


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